Inheritance, Genes, and Chromosomes

Inheritance, Genes, and Chromosomes
8 Inheritance, Genes, and Chromosomes

Chapter 8 Inheritance, Genes, and Chromosomes
Key Concepts 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws 8.2 Alleles and Genes Interact to Produce Phenotypes 8.3 Genes Are Carried on Chromosomes 8.4 Prokaryotes Can Exchange Genetic Material

Chapter 8 Opening Question
How is hemophilia inherited, and why is it most frequent in males?

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws
Breeding of plants and animals had yielded two hypotheses by the mid-19th century: Blending inheritance—gametes contained determinants (genes) that blended when gametes fused during fertilization Particulate inheritance—each determinant was physically distinct and remained intact during fertilization

In the 1860s, the monk Gregor Mendel used the scientific method in studies of garden peas that clearly supported the particulate hypothesis. Garden peas were easy to grow and manipulate: their flowers have both male and female sex organs (pistils and stamens) that produce gametes.

Peas normally self-fertilize, but male organs can be removed to allow fertilization with pollen from other flowers. 6

Traits—forms of a character (e.g., purple flowers, wrinkled seeds)
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Characters—observable physical features (e.g., flower color, seed shape) Traits—forms of a character (e.g., purple flowers, wrinkled seeds) Mendel worked with true-breeding varieties— when plants of the same variety are crossed, all offspring have characters with the same traits.

Parental generation = P
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Parental generation = P Resulting offspring = first filial generation, or F1 If F1 plants self-pollinate, they produce the second filial generation, or F2. “Hybrid” refers to offspring of crosses between organisms differing in one or more characters.

In the 1st experiment, Mendel crossed plants differing in just one character, producing monohybrids in the F1 generation. The monohybrids were then allowed to self- pollinate to form the F2 generation—a monohybrid cross. 9

Figure 8.1 Mendel’s Monohybrid Experiments (Part 1)
Figure 8.1 Mendel’s Monohybrid Experiments Mendel performed crosses with pea plants and carefully analyzed the outcomes to show that genetic determinants are particulate.a [a See

Figure 8.1 Mendel’s Monohybrid Experiments (Part 2)
Figure 8.1 Mendel’s Monohybrid Experiments Mendel performed crosses with pea plants and carefully analyzed the outcomes to show that genetic determinants are particulate.a [a See

Mendel’s findings led to a rejection of the blending hypothesis.
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws One trait of each pair disappeared in the F1 generation and reappeared in F2 . Mendel’s findings led to a rejection of the blending hypothesis. The F1 offspring were not a blend of the parental traits, but only one trait was present. The trait did not disappear in the F2 generation.

All seven crosses between varieties with contrasting traits gave the same kind of data. Mendel called the trait that appeared in the F1 and was more abundant in the F2 the dominant trait, and the other trait recessive. The ratio of dominant to recessive in the F2 was about 3:1. 13

Each plant has two genes for each character, one from each parent.
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Mendel proposed that the genetic determinants (genes) occur in pairs and are segregated in the gametes. Each plant has two genes for each character, one from each parent. We now use the term diploid to describe having two copies of a gene, and haploid as having one copy.

Mendel concluded that each gamete contains one copy of each gene (haploid), the resulting zygote contains two copies (diploid), because it is produced by the fusion of two gametes. Different traits arise because there can be different forms of a gene—now called alleles—for a particular character.

Phenotype—the physical appearance of an organism (e.g., round seeds)
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Phenotype—the physical appearance of an organism (e.g., round seeds) Genotype—the genetic makeup (e.g., Rr) Capital letters designate the dominant allele, lower case designate the recessive. Round seeds can be the result of two different genotypes—RR or Rr.

True-breeding individuals have two copies of the same allele—they are homozygous for the allele (e.g., rr). Heterozygous individuals have two different alleles (e.g., Rr).

Figure 8.2 Mendel’s Explanation of Inheritance
Figure 8.2 Mendel’s Explanation of Inheritance Mendel concluded that inheritance depends on discrete factors from each parent that do not blend in the offspring.

Each gamete receives only one copy.
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Mendel’s first law: The law of segregation states that the two copies of a gene separate when an individual makes gametes. Each gamete receives only one copy. Gametes from an RR individual will all be R, gametes from an rr individual will all be r, and their offspring (F1) will all be Rr.

In the F2 generation, gametes will be R or r.
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws In the F2 generation, gametes will be R or r. Genotypes in the F2 generation can be predicted using a Punnett square, which considers all possible gamete combinations.

The Punnett square predicts a 3:1 ratio of phenotypes in the F2 generation. Mendel’s experimental values were remarkably close to this for all seven of the character traits he compared.

Genes are now known to be relatively short sequences of DNA found on the much longer DNA molecules that make up chromosomes. Today, we can picture the different alleles of a gene segregating as chromosomes separate during meiosis I.

Figure 8.3 Meiosis Accounts for the Segregation of Alleles (Part 1)
Figure 8.3 Meiosis Accounts for the Segregation of Alleles Although Mendel had no knowledge of chromosomes or meiosis, we now know that a pair of alleles resides on homologous chromosomes, and that those alleles segregate during meiosis.

Figure 8.3 Meiosis Accounts for the Segregation of Alleles (Part 2)
Figure 8.3 Meiosis Accounts for the Segregation of Alleles Although Mendel had no knowledge of chromosomes or meiosis, we now know that a pair of alleles resides on homologous chromosomes, and that those alleles segregate during meiosis.

Genes determine phenotypes mostly by producing proteins with particular functions. In many cases a dominant gene is expressed, and the recessive gene is mutated so that it is no longer expressed or produces a non-functional protein. The wrinkled pea phenotype is due to a lack of starch branching enzyme 1 (SBE1), essential for starch synthesis.

Mendel verified his hypotheses by doing test crosses:
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Mendel verified his hypotheses by doing test crosses: Cross F1 with known homozygote which has the recessive genotype (e.g., rr). If the F1 individual was homozygous, all offspring will show the dominant trait. If the F1 was heterozygous, half of the offspring will have the recessive trait.

Figure 8.4 Homozygous or Heterozygous? (Part 1)
Figure 8.4 Homozygous or Heterozygous? An individual with a dominant phenotype may have either a homozygous or a heterozygous genotype. The test cross determines which.a [a See

Figure 8.4 Homozygous or Heterozygous? An individual with a dominant phenotype may have either a homozygous or a heterozygous genotype. The test cross determines whichwhich.a [a See

Figure 8.4 Homozygous or Heterozygous? An individual with a dominant phenotype may have either a homozygous or a heterozygous genotype. The test cross determines which.a [a See

He crossed peas that differed in two characters—seed shape and color
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Mendel’s next experiments tested whether genes are distributed independently in the offspring. He crossed peas that differed in two characters—seed shape and color True-breeding parents: RRYY—round yellow seeds rryy—wrinkled green seeds

F1 generation is RrYy—all round yellow.
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws F1 generation is RrYy—all round yellow. Crossing the F1 generation (all identical double heterozygotes) is a dihybrid cross. Mendel asked whether, in the gametes produced by RrYy, the traits would be linked or segregate independently.

Two possible outcomes of the dihybrid cross:
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Two possible outcomes of the dihybrid cross: If the alleles were linked, gametes would be RY or ry; F2 would have three times more round yellow than wrinkled green. The ratio would be 3:1 If independent, gametes could be RY, ry, Ry, or rY. F2 generation would have nine different genotypes, and the four phenotypes would be in a 9:3:3:1 ratio.

Figure 8.5 Independent Assortment
Figure 8.5 Independent Assortment The 16 possible combinations of gametes in this dihybrid cross result in nine different genotypes. Because R and Y are dominant over r and y, respectively, the nine genotypes result in four phenotypes in a ratio of 9:3:3:1. These results show that the two genes segregate independently.

Mendel’s dihybrid crosses supported the second prediction.
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Mendel’s dihybrid crosses supported the second prediction. Mendel’s second law is the law of independent assortment: Alleles of different genes assort independently during gamete formation. This law does not always apply to genes near each other on the same chromosome, but chromosomes do segregate independently.

Figure 8.6 Meiosis Accounts for Independent Assortment of Alleles (Part 1)
Figure 8.6 Meiosis Accounts for Independent Assortment of Alleles We now know that copies of genes on different chromosomes are segregated independently during metaphase I of meiosis. Thus a parent of genotype RrYy can form gametes with four different genotypes.

Figure 8.6 Meiosis Accounts for Independent Assortment of Alleles (Part 2)
Figure 8.6 Meiosis Accounts for Independent Assortment of Alleles We now know that copies of genes on different chromosomes are segregated independently during metaphase I of meiosis. Thus a parent of genotype RrYy can form gametes with four different genotypes.

One reason Mendel was successful was his use of large sample sizes.
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws One reason Mendel was successful was his use of large sample sizes. By counting many progeny from each cross, he was able to see clear patterns. After his work became recognized, geneticists began using probability calculations to predict expected ratios of genotypes and phenotypes and statistics to determine whether actual results matched the predictions.

If an event is certain to happen, its probability = 1
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Probability If an event is certain to happen, its probability = 1 If an event cannot possibly happen, its probability = 0 All other events have a probability between 0 and 1

The probability that both coins will come up heads is:
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Two coin tosses are independent events; each will come up heads ½ of the time. The probability that both coins will come up heads is: ½ × ½ = ¼ Multiplication rule: the probability of two independent outcomes occurring together is found by multiplying the individual probabilities.

Figure 8.7 Using Probability Calculations in Genetics
Figure 8.7 Using Probability Calculations in Genetics Like the results of a coin toss, the probability of any given combination of alleles appearing in the offspring of a cross can be obtained by multiplying the probabilities of each event. Since a heterozygote can be formed in two ways, these two probabilities are added together.

Probability that offspring will have the rr genotype is the same.
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws In a monohybrid cross: After self-pollination of an F1 Rr, the probability that the F2 offspring will have the genotype RR is ½ × ½ = ¼ Probability that offspring will have the rr genotype is the same.

Probability that offspring will be heterozygous: There are two ways to get a heterozygote— Rr or rR. Addition rule: probability of an event that can occur in two or more different ways is the sum of the individual probabilities of those ways: ¼ + ¼ = ½

Human pedigrees can show Mendel’s laws.
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Human pedigrees can show Mendel’s laws. Humans have few offspring; pedigrees do not show the clear proportions that the pea plants showed. But pedigrees can show patterns and can be used to determine whether a rare allele is dominant or recessive.

Pattern of inheritance if a rare allele is dominant:
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Pattern of inheritance if a rare allele is dominant: Every person with the abnormal phenotype has an affected parent. Either all (if homozygous parent) or half (if heterozygous parent) of offspring in an affected family are affected.

Figure 8.8 Pedigree Analysis and Inheritance (Part 1)
Figure 8.8 Pedigree Analysis and Inheritance (A) This pedigree represents a family affected by Huntington’s disease, which results from a rare dominant allele. Everyone who inherits this allele is potentially affected. (B) The family in this pedigree carries the allele for albinism, a recessive trait. Because the trait is recessive, heterozygotes do not have the albino phenotype, but they can pass the allele on to their offspring. Affected persons must inherit the allele from two heterozygous parents, or (rarely) from one homozygous recessive and one heterozygous parent, or (very rarely) two homozygous recessive parents. In this family, the heterozygous parents in generation III are cousins; however, the same result would occur if the parents were unrelated.

Pattern of inheritance if the rare allele is recessive:
Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws Pattern of inheritance if the rare allele is recessive: Affected people often have two unaffected parents. In an affected family, one-fourth of children of unaffected parents are affected. The parents are heterozygous carriers.

Figure 8.8 Pedigree Analysis and Inheritance (Part 2)
Figure 8.8 Pedigree Analysis and Inheritance (A) This pedigree represents a family affected by Huntington’s disease, which results from a rare dominant allele. Everyone who inherits this allele is potentially affected. (B) The family in this pedigree carries the allele for albinism, a recessive trait. Because the trait is recessive, heterozygotes do not have the albino phenotype, but they can pass the allele on to their offspring. Affected persons must inherit the allele from two heterozygous parents, or (rarely) from one homozygous recessive and one heterozygous parent, or (very rarely) two homozygous recessive parents. In this family, the heterozygous parents in generation III are cousins; however, the same result would occur if the parents were unrelated.

Concept 8.2 Alleles and Genes Interact to Produce Phenotypes
New alleles arise by mutations—rare, stable, inherited changes in the genetic material. Wild type is the allele present in most of the population. Other alleles of that gene are mutant alleles. A gene with a wild-type allele that is present less than 99% of the time is called polymorphic.

One gene may have more than two alleles. Multiple alleles increase the number of possible phenotypes and may show a hierarchy of dominance in heterozygotes. Example: coat color in rabbits

Figure 8.9 Multiple Alleles for Coat Color in Rabbits
Figure 8.9 Multiple Alleles for Coat Color in Rabbits These photographs show the phenotypes conferred by four alleles of the C gene for coat color in rabbits. Different combinations of two alleles give different coat colors and pigment distributions.

Incomplete dominance Some alleles are neither dominant nor recessive, and heterozygotes have an intermediate phenotype. Example: flower color in snapdragons

Figure 8.10 Incomplete Dominance Follows Mendel’s Laws
Figure Incomplete Dominance Follows Mendel’s Laws An intermediate phenotype can occur in heterozygotes when neither allele is dominant. The heterozygous phenotype (here, pink flowers) may give the appearance of a blended trait, but the traits of the parental generation reappear in their original forms in succeeding generations, as predicted by Mendel’s laws of inheritance.

Codominance Two alleles of a gene produce both phenotypes in the heterozygote. Example: ABO blood group system has three alleles of the gene: IA, IB, and IO. IA and IB are codominant.

Figure 8.11 ABO Blood Groups Are Important in Transfusions
Figure ABO Blood Groups Are Important in Transfusions The table shows the results of mixing red blood cells of types A, B, AB, and O with serum containing anti-A or anti-B antibodies. As you look down the two columns on the right, note that each of the types, when mixed separately with anti-A and with anti-B, gives a unique pair of results. This is the basic method by which blood is typed.

Epistasis Phenotypic expression of one gene is influenced by another gene. Example: Coat color in Labrador retrievers Allele B (black) is dominant to b (brown) Allele E (pigment deposition) is dominant to e (no pigment deposition—hair is yellow) Gene E determines the phenotypic expression of gene B.

Figure 8.12 Genes Interact Epistatically
Figure Genes Interact Epistatically Epistasis occurs when one gene alters the phenotypic effect of another gene. In Labrador retrievers, the E gene determines the expression of the B gene.

Hybrid vigor (heterosis): hybrid offspring grow larger, produce more seeds, etc, than the parental varieties.

“Vigor” is a complex trait; most complex phenotypes are determined by multiple genes. Most are quantitative traits: they must be measured, rather than assessed qualitatively (e.g., grain yields).

Genotype and environment often interact to determine phenotype. Example: point restriction coat patterns

These rabbits and cats have a mutant allele for the coat color gene. The enzyme encoded by the gene is inactive at temperatures above about 35°C. The extremities are cooler than the main body (around 25°C), so the fur on these regions is dark.

Two parameters describe the effects of genes and environment on phenotype: Penetrance: proportion of individuals with a certain genotype that show the expected phenotype Expressivity: degree to which genotype is expressed in an individual

Many people, but not all, who inherit a mutant allele of the gene BRCA1 develop breast cancer. The mutation is said to be incompletely penetrant. A woman with the mutant allele may get both breast and ovarian cancer, but another woman may only get breast cancer. The mutation is said to have variable expressivity.

Heritability: relative contribution of genetic versus environmental factors to variation in a character in a population Example: human height

Heritability varies from 0 to 1. For human height, heritability varies from about 0.65 to 0.8. If heritability is 0.65, 65% of the variation in height is due to genetic factors, 35% is due to environmental effects. Heritability estimates apply only to variation within populations, not individuals.

Concept 8.3 Genes Are Carried on Chromosomes
A gene is a sequence of DNA that resides at a particular site on a chromosome—the locus (plural loci). Genetic linkage of genes on a single chromosome can alter their pattern of inheritance from those described by Mendel’s law.

Genetic linkage was discovered by Thomas Hunt Morgan, using the fruit fly Drosophila melanogaster. Much genetic research has been done with Drosophila, which is considered a model organism because of its small size, ease of breeding, and short generation time.

Some crosses performed with Drosophila did not yield expected ratios according to the law of independent assortment. Instead, some alleles for body color and wing shape were inherited together. Morgan hypothesized that the two loci were linked on the same chromosome and could not assort independently.

Figure 8.13 Some Alleles Do Not Assort Independently (Part 1)
Figure Some Alleles Do Not Assort Independently Morgan’s studies showed that the genes for body color and wing size in Drosophila are linked, so that their alleles do not assort independently.a [a T. H. Morgan Science 36: 719–720.]

Figure 8.13 Some Alleles Do Not Assort Independently (Part 2)
Figure Some Alleles Do Not Assort Independently Morgan’s studies showed that the genes for body color and wing size in Drosophila are linked, so that their alleles do not assort independently.a [a T. H. Morgan Science 36: 719–720.]

But if linkage were absolute, the F1 would only have the two parental phenotypes in a 1:1 ratio. Some of the offspring had recombinant phenotypes, resulting from crossing over during prophase I in meiosis. Two of the four chromatids in the tetrad exchange chromosome segments.

Figure 8.14 Crossing Over Results in Genetic Recombination
Figure Crossing Over Results in Genetic Recombination Recombination accounts for why linked alleles are not always inherited together. Alleles at different loci on the same chromosome can be recombined by crossing over and then being separated from one another. Such recombination occurs during prophase I of meiosis.

Recombinant offspring generally appear in proportions related to the recombination frequency between the two genes: Calculated by dividing the number of recombinant progeny by the total number of progeny. Recombinant frequencies are higher for loci that are farther apart on the chromosome.

Figure 8.15 Recombination Frequencies
Figure Recombination Frequencies The frequency of recombinant offspring (those with a phenotype different from either parent) can be calculated.

Recombinant frequencies can be used to infer the locations of genes along a chromosome and make a genetic map.

Fruit flies have one pair of sex chromosomes (involved in sex determination) and 3 pairs of autosomes (all the other chromosomes).

Males have X and Y sex chromosomes that are different in size, and many genes on the X chromosome are not present on the Y. Thus, males have only one copy of some genes. A gene that is present as a single copy in a diploid organism is called hemizygous.

Morgan’s experiments showed that the gene for eye color was carried on the X chromosome. The wild-type is red (R), the mutant allele is white (r).

Figure 8.16 A Gene for Eye Color Is Carried on the Drosophila X Chromosome
Figure A Gene for Eye Color Is Carried on the Drosophila X Chromosome Morgan demonstrated that a mutant allele that causes white eyes in Drosophila is carried on the X chromosome. Note that in this case, the reciprocal crosses do not have the same results.

The term sex-linked inheritance refers to inheritance of a gene that is carried on a sex chromosome. In mammals, the X chromosome is larger and carries more genes that the Y; most sex- linked inheritance involves genes that are carried on the X chromosome.

Figure 8.17 Red–Green Color Blindness Is Carried on the Human X Chromosome
Figure Red–Green Color Blindness Is Carried on the Human X Chromosome The mutant allele for red–green color blindness is expressed as an X-linked recessive trait, and therefore is always expressed in males when they carry that allele.

Patterns in X-linked recessive phenotypes: They appear much more often in males than females. A male with the mutation can pass it only to daughters. Daughters who receive one X-linked mutation are heterozygous carriers. Mutant phenotype can skip a generation if it passes from a male to his daughter (normal phenotype) and then to her son.

Mitochondria and plastids also have a small chromosome. Egg cells have abundant cytoplasm and organelles, but the only part of the sperm that takes part in fertilization is the nucleus. So, mitochondria and plastids are inherited only from the mother. Inheritance of organelles and their genes is thus non-Mendelian and is called maternal, or cytoplasmic, inheritance.

Some organelle genes are important for organelle assembly and function, and mutations can have large effects. In plants, a mutation that affects proteins that assemble chlorophyll molecules into photosystems results in all white plants.

Figure 8.18 Cytoplasmic Inheritance
Figure Cytoplasmic Inheritance In four o’clock plants, leaf color is inherited through the female plant only. In the parent plant with some white leaves, the white leaf color is caused by a chloroplast mutation that occurred during the life of the plant; the leaves that formed before the mutation occurred are green. When this plant is used as a pollen donor in a cross with an all green plant, the offspring are all green. But when the same plant is used as an egg donor, the offspring inherit the mutation cytoplasmically and are entirely white.

Concept 8.4 Prokaryotes Can Exchange Genetic Material
Prokaryotes reproduce asexually by binary fission, which gives rise to genetically identical progeny. But they can also exchange genetic material. Transfer of genes from one individual to another without sexual reproduction is called horizontal or lateral gene transfer. Along with mutation, this process generates genetic diversity among prokaryotes.

Bacteria exchange genes by bacterial conjugation. A projection called the sex pilus initiates contact between cells. Genetic material then passes from the donor to the recipient through a conjugation tube. There is no reciprocal transfer of DNA.

Figure 8.19 Bacterial Conjugation and Recombination (Part 1)
Figure Bacterial Conjugation and Recombination (A) A sex pilus draws two bacteria into close contact, so that a cytoplasmic bridge (conjugation tube) can form. DNA is transferred from the donor cell to the recipient cell via the conjugation tube. (B) DNA from a donor cell can become incorporated into a recipient cell’s chromosome through crossing over.

Figure 8.19 Bacterial Conjugation and Recombination (Part 2)
Figure Bacterial Conjugation and Recombination (A) A sex pilus draws two bacteria into close contact, so that a cytoplasmic bridge (conjugation tube) can form. DNA is transferred from the donor cell to the recipient cell via the conjugation tube. (B) DNA from a donor cell can become incorporated into a recipient cell’s chromosome through crossing over.

In the recipient cell, the donor DNA lines up with the recipient’s DNA and crossing over occurs. Genes from the donor are incorporated into the recipient’s genome and are passed on to its progeny.

Many bacteria also have plasmids—small circular DNA molecules that replicate independently. Plasmids often have: Genes for unusual metabolic tasks, such as breaking down hydrocarbons Genes for antibiotic resistance Plasmids can move between cells during conjugation.

Figure 8.20 Gene Transfer by Plasmids
Figure Gene Transfer by Plasmids When plasmids enter a cell via conjugation, their genes can be expressed in the recipient cell.

Discovery of antibiotics such as penicillin greatly reduced many lethal infections. But over time, some bacteria acquired mutations that made them resistant. With a selective advantage, as penicillin use spread, so did the resistant bacteria. Scientists designed variants of penicillin; but new strains of resistant bacteria also develop. This “arms race” still continues.

Answer to Opening Question
In hemophilia, the mutant gene for the clotting factor is recessive, and is carried on the X chromosome. Affected males inherit their single X chromosome from their mothers. If they receive their mother’s recessive mutant chromosome, they cannot produce the clotting factor and have hemophilia.

Figure 8.21 Sex Linkage in Royal Families of Europe
Figure Sex Linkage in Royal Families of Europe England’s Queen Victoria passed an X chromosome carrying the mutant allele for hemophilia to three of her children.

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