Chapter 12: Patterns of Inheritance Fig. 12.2 Chapter 12: Patterns of Inheritance Chapter 13: Chromosomes, Mapping, and the Meiosis-Inheritance Connection

Patterns of Inheritance Chapter 12

Early Ideas of Heredity Before the 20th century, 2 concepts were the basis for ideas about heredity: heredity occurs within species traits are transmitted directly from parent to offspring Led to the belief that inheritance is a matter of blending traits from the parents.

Early Ideas of Heredity Botanists in the 18th and 19th centuries produced hybrid plants. When the hybrids were crossed with each other, some of the offspring resembled the original strains, rather than the hybrid strains. This evidence contradicted the idea that traits are directly passed from parent to offspring.

Fig. 12.3-3

Early Ideas of Heredity Gregor Mendel Studied the pea plant: other research showed that pea hybrids could be produced many pea varieties were available peas are small plants and easy to grow peas can self-fertilize or be cross-fertilized

Early Ideas of Heredity Gregor Mendel’s experimental method: Produce true-breeding strains for each trait he was studying Cross-fertilize true-breeding strains having alternate forms of a trait perform reciprocal crosses as well Allow the hybrid offspring to self-fertilize and count the number of offspring showing each form of the trait

Fig. 12.3-1

Fig. 12.3-2

Monohybrid Crosses Monohybrid cross: a cross to study only 2 variations of a single trait Mendel produced true-breeding pea strains for 7 different traits each trait had 2 alternate forms (variations) flower color, pea color, pea shape... Mendel cross-fertilized the 2 true-breeding strains for each trait

Monohybrid Crosses F1 generation (1st filial generation): offspring produced by crossing 2 true- breeding strains For every trait Mendel studied, all F1 plants resembled only 1 parent No plants with characteristics intermediate between the 2 parents were produced

Monohybrid Crosses F2 generation (2nd filial generation): the offspring resulting from the self-fertilization of F1 plants

Monohybrid Crosses F2 generation (2nd filial generation): the offspring resulting from the self-fertilization of F1 plants Dominant traits: the form of each trait most commonly expressed in the F1 plants Recessive traits: the form of the trait not seen in the F1 plants; hidden traits

Monohybrid Crosses F2 plants exhibited both forms of the trait in a very specific pattern: ¾ plants with the dominant form ¼ plant with the recessive form The dominant to recessive ratio was 3 : 1. Mendel discovered the ratio is actually: 1 true-breeding dominant plant 2 not-true-breeding dominant plants 1 true-breeding recessive plant

Monohybrid Crosses Gene: information for a trait passed from parent to offspring Alleles: alternate forms of a gene Homozygous: having 2 of the same allele Heterozygous: having 2 different alleles

Monohybrid Crosses Genotype: total set of alleles of an individual PP = homozygous dominant Pp = heterozygous pp = homozygous recessive Phenotype: outward appearance of an individual physical manifestation of genotype

Monohybrid Crosses Principle of Segregation: Two alleles for a gene segregate during gamete formation homologous chromosome separation during meiosis Alleles are re-paired at random, one from each parent, during fertilization.

Monohybrid Crosses Some human traits are controlled by a single gene. some of these exhibit dominant inheritance some of these exhibit recessive inheritance Carriers are heterozygous Pedigree analysis is used to track inheritance patterns in families.

Dihybrid Crosses Dihybrid cross: examination of 2 separate traits in a single cross for example: RR YY x rryy The F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait.

Dihybrid Crosses The F2 generation is produced by crossing members of the F1 generation with each other or allowing self-fertilization of the F1. for example RrYy x RrYy The F2 generation shows all four possible phenotypes in a set ratio: 9 : 3 : 3 : 1

Dihybrid Crosses Principle of Independent Assortment: In a dihybrid cross, the alleles of each gene assort independently.

Probability – Predicting Results Rule of addition: the probability of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities. When crossing Pp x Pp, the probability of producing Pp offspring is: probability of obtaining Pp (1/4), PLUS probability of obtaining pP (1/4) ¼ + ¼ = ½

Probability – Predicting Results Rule of multiplication: the probability of 2 independent events occurring simultaneously is the PRODUCT of their individual probabilities. When crossing Rr Yy x RrYy, the probability of obtaining rr yy offspring is: probability of obtaiing rr = ¼ probability of obtaining yy = ¼ probability of rr yy = ¼ x ¼ = 1/16

Testcross Testcross: a cross used to determine the genotype of an individual with dominant phenotype cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp) the phenotypic ratios among offspring are different, depending on the genotype of the unknown parent

Extensions to Mendel Mendel’s model of inheritance assumes: each trait is controlled by a single gene each gene has only 2 alleles there is a clear dominant-recessive relationship between the alleles Most genes do not meet these criteria. Other types of inheritance: Polygenic, Pleiotropy, Incomplete dominance, codominance

Extensions to Mendel Polygenic inheritance occurs when multiple genes are involved in controlling the phenotype of a trait. The phenotype is an accumulation of contributions by multiple genes. These traits show continuous variation and are referred to as quantitative traits. For example – human height

Extensions to Mendel Pleiotropy refers to an allele which has more than one effect on the phenotype. This can be seen in human diseases such as cystic fibrosis or sickle cell anemia. In these diseases, multiple symptoms can be traced back to one defective allele.

Extensions to Mendel Incomplete dominance: the heterozygote is intermediate in phenotype between the 2 homozygotes. Codominance: the heterozygote shows some aspect of the phenotypes of both homozygotes.

Extensions to Mendel The human ABO blood group system: -multiple alleles: there are 3 alleles of the I gene (IA, IB, and i) -codominance: IA and IB are dominant to i but codominant to each other

Extensions to Mendel The expression of some genes can be influenced by the environment. for example: coat color in Himalayan rabbits and Siamese cats an allele produces an enzyme that allows pigment production only at temperatures below 30oC

Extensions to Mendel

Extensions to Mendel The products of some genes interact with each other and influence the phenotype of the individual. Epistasis: one gene can interfere with the expression of another gene

Chromosomes, Mapping, and the Meiosis-Inheritance Connection Chapter 13

Chromosome Theory Chromosomal theory of inheritance developed in 1902 by Walter Sutton proposed that genes are present on chromosomes based on observations that homologous chromosomes pair with each other during meiosis supporting evidence was provided by work with fruit flies

Chromosome Theory T.H. Morgan isolated a mutant white-eyed Drosophila Red-eyed female X white-eyed male gave a F1 generation of all red eye Morgan concluded that red eyes are dominant

Fig. 13.1

Chromosome Theory Morgan crossed F1 females X F1 males F2 generation contained red and white- eyed flies but all white-eyed flies were male Testcross of a F1 female with a white-eyed male showed the viability of white-eyed females Morgan concluded that the eye color gene is linked to the X chromosome

Sex Chromosomes Sex determination in Drosophila is based on the number of X chromosomes 2 X chromosomes = female 1 X and 1 Y chromosome = male Sex determination in humans is based on the presence of a Y chromosome having a Y chromosome (XY) = male

Sex Chromosomes Sex-linked traits: traits controlled by genes present on the X chromosome Sex-linked traits show inheritance patterns different than those of genes on autosomes. In many organisms, the Y chromosome is greatly reduced or inactive. Genes on the X chromosome are present in only 1 copy in males

Sex Chromosomes Dosage compensation ensures an equal expression of genes from the sex chromosomes even though females have 2 X chromosomes and males have only 1. In each female cell, 1 X chromosome is inactivated and is highly condensed into a Barr body. Females heterozygous for genes on the X chromosome are genetic mosaics. Phenotype depends on which X chromosome inactivated

Chromosome Theory Exceptions Mitochondria and chloroplasts contain genes. Traits controlled by these genes do not follow the chromosomal theory of inheritance Genes from mitochondria and chloroplasts are often passed to the offspring by only one parent

Chromosome Theory Exceptions Maternal inheritance: uniparental (one- parent) inheritance from the mother The mitochondria in a zygote are from the egg cell; no mitochondria come from the sperm during fertilization In plants, the chloroplasts are often inherited from the mother, although this is species dependent

Genetic Mapping The science of determining the location of a gene on a chromosome Early geneticists realized that they could obtain information about the distance between genes on a chromosome. This is genetic mapping Mapping is based on genetic recombination (crossing over) between genes.

Genetic Mapping To determine the distance between genes: dihybrid organisms are testcrossed offspring resembling the dihybrid parent result from homologues that were not involved in the crossover offspring resulting from a crossover are called recombinant progeny

Genetic Mapping The distance between genes is proportional to the frequency of recombination events. recombination recombinant progeny frequency total progeny 1% recombination = 1 map unit (m.u.) 1 map unit = 1 centimorgan (cM) =

Genetic Mapping Determining the order of genes can be done with a three-point testcross The frequency of double crossovers is the product of the probabilities of each individual crossover Therefore, the classes of offspring with the lowest numbers represent the double crossovers and allow the gene order to be determined

Human Genetic Disorders Some human genetic disorders are caused by altered proteins: the altered protein is encoded by a mutated DNA sequence the altered protein does not function correctly, causing a change to the phenotype the protein can be altered at only a single amino acid (e.g. sickle cell anemia)

Fig. 13.11

Human Genetic Disorders Some genetic disorders are caused by a change in the number of chromosomes. Nondisjunction during meiosis can create gametes having one too many or one too few chromosomes Fertilization of these gametes creates trisomic or monosomic individuals Down syndrome is trisomy of chromosome 21

Human Genetic Disorders Nondisjunction of sex chromosomes can result in: XXX triple-X females XXY males (Klinefelter syndrome) XO females (Turner syndrome) OY nonviable zygotes XYY males (Jacob syndrome)

Sex Determination in Humans The sex chromosomes, X and Y, are a homologous pair: this pair is unique because X and Y carry different sets of genes the Y chromosome has genes that determine maleness the X chromosome has a variety of genes on it XX = female; XY = male X_ = female; _Y = does not survive XXY = male 82

Sex Determination in Humans Genotype: X__ Sex: Female Phenotype: Turner’s Syndrome short stature (less than 5’) ‘webbed’ neck and other physical characteristics infertility 83

Sex Determination in Humans Genotype: XXX Sex: Female Phenotype: ‘Super-females’, metafemales tall stature longer legs and torso may have learning disabilities or emotionally underdeveloped commonly labeled as ‘trouble makers’ in school 84

Sex Determination in Humans Genotype: XYY Sex: Male Phenotype: ‘Super-males’ produce higher levels of testosterone may be taller than average no known significant abnormalities 85

Sex Determination in Humans Genotype: XXY or XXXY Sex: Male Phenotype: Klinefelter Syndrome produce very little testosterone taller and more overweight than average may have feminine characteristics sterile or nearly sterile most have normal cognitive abilities can be treated with testosterone early in life 86

Table 13.2-1

Table 13.2-2

Human Genetic Disorders Genomic imprinting occurs when the phenotype exhibited by a particular allele depends on which parent contributed the allele to the offspring A specific partial deletion of chromosome 15 results in: Prader-Willi syndrome if the chromosome is from the father Angelman syndrome if it’s from the mother

Human Genetic Disorders Genetic counseling can use pedigree analysis to determine the probability of genetic disorders in the offspring. Some genetic disorders can be diagnosed during pregnancy: amniocentesis collects fetal cells from the amniotic fluid for examination chorionic villi sampling collects cells from the placenta for examination