GENES AND HEREDITY

What we are asked to teach our students The Standards What we are asked to teach our students

7th Grade Life Science Major Concepts/ Skills Diversity of living Dichotomous key/classify (6 Kingdoms) Structure and function of cells Tissues, organs, and organ systems Purpose of major human body organ systems Heredity, genes, and successive generations Ecosystems Cycling of matter and energy Biological evolution Natural selection and fossil record

S7L3. Students will recognize how biological traits are passed on to successive generations. a. Explain the role of genes and chromosomes in the process of inheriting a specific trait. b. Compare and contrast that organisms reproduce asexually and sexually (bacteria, protists, fungi, plants & animals). c. Recognize that selective breeding can produce plants or animals with desired traits.

Translating the Standards What are we trying to help the students understand?

Students need to know what genes and chromosomes are a. Explain the role of genes and chromosomes in the process of inheriting a specific trait Students need to know what genes and chromosomes are What is their structure? How genes code for the proteins that result in interaction with the environment Students need to know the difference between the genotype and phenotype of an organism Students need to know the basics of Mendelian genetics and inheritance

b. Compare and contrast that organisms reproduce asexually and sexually (bacteria, protists, fungi, plants & animals). Students need to know that bacteria reproduce asexually by binary fission Students need to know that mitosis is a type of nuclear and cell division that is used both in growth and asexual reproduction Students need to know that meiosis is a type of nuclear and cell division that is used to make the sperm and eggs used in sexual reproduction

c. Recognize that selective breeding can produce plants or animals with desired traits. For students to understand selective breeding (artificial selection) they need to recognize that: Genotype controls phenotype and that a desirable trait is a phenotype A population of organisms usually has large genetic variation and reflected in a variety of forms of a trait The only difference between artificial selection and natural selection is the source of the selective pressure

Major Topics in Genes and Heredity The Inheritance of Traits Mendelian Genetics: When the Role of Genes Is Clear Quantitative Genetics: When Genes and Environment Interact Genes, Environment, and the Individual

The Inheritance of Traits Most children are similar to their parents Children tend to be similar to siblings Each child is a combination of parental traits The combination of paternal traits and maternal traits is unique for each individual child

The human life cycle gametes (a male sperm cell + a female egg cell) fuse during fertilization to form a single celled zygote, or embryo the embryo grows by cell division in mitosis the embryo grows into a child the child matures into an adult

Figure 6.1 The human life cycle. A human baby forms from the fusion of an egg cell from its mother and a sperm cell from its father. The single cell that results from this fusion will grow and divide into trillions of cells, each carrying the same information.

Figure 6.1 The human life cycle. A human baby forms from the fusion of an egg cell from its mother and a sperm cell from its father. The single cell that results from this fusion will grow and divide into trillions of cells, each carrying the same information.

Genes Most genes are segments of DNA that carry information about how to make proteins Structural proteins – for things like hair Functional proteins – for things like breaking down lactose

Genes All cells have the same genes Only certain genes are active in a single cell Heart cells and eye cells have genes for the protein rhodopsin, which helps to detect light This is only produced in eye cells, not heart cells

Genes and Chromosomes DNA is sort of like an instruction manual that shows how to build and maintain a living organism…

Figure 6.2 Genes as words in an instruction manual. Different words from the manual are used in different parts of the body, and even when the same words are used, they are often used in distinctive combinations. In this way, the manual can provide instructions for making and operating the variety of body parts we possess.

Genes Are on Chromosomes The genes are located on the chromosomes The number of chromosomes depends on the organism Bacteria – one circular chromosome Humans – 23 homologous pairs of linear chromosomes

Genes Are on Chromosomes Each of the 23 pairs of chromosomes is a homologous pair that carry the same gene For each homologous pair, one came from mom and the other from dad

Figure 6.3 The formation of different alleles. (a) Each parent provides a complete set of instructions to each offspring.

Gene Variation Is Caused by Mutation Genes on a homologous pair are the same, but the exact information may not be the same Sometimes errors or mutations in gene copies can cause somewhat different proteins to be produced Different gene versions are called alleles

Figure 6.3 The formation of different alleles. (b) The instructions are first copied, and different alleles for a gene may form as a result of copying errors. Some of these misspellings do not change the meaning of the word, but some may result in altered meanings or have no meaning at all.

Diversity in Offspring The combination from the parents creates the individual traits of each child Environment also plays a role, but differing alleles from parents are the primary reason that non-twin siblings are not identical

Diversity in Offspring Non-twin siblings: The combination each individual receives depended on the gametes that were part of the fertilization event Remember that each gamete has 1 copy of each homologous pair

Segregation When a gamete is formed, the homologous pairs are separated and segregated into separate gametes (this is called the law of segregation) This results in gametes with only 23 chromosomes 1 of each homologous pair

Independent Assortment Due to independent assortment, parents contribute a unique subset of alleles to each of their non-identical twin offspring Since each gamete is produced independently, the combination of genes is unique

Figure 6.4 Each egg and sperm is unique. This figure uses the instruction manual analogy to show how a single man can produce an enormous diversity of sperm. The same process of independent assortment results in enormous diversity in eggs as well. The cells that are the source of a man’s sperm carry 2 of each chromosome— that is, 2 full copies of the instruction manual—1 set from his mom, the other from his dad. When a sperm cell is produced, it ends up with only 1 copy of each page. Since each sperm is produced independently, the set of pages in each sperm will be a unique combination of the pages that the man inherited from his mom and dad.

Diversity in Offspring That means a unique egg will be fertilized by a unique sperm to produce a unique child For each gene, there is a 50% chance of having the same allele as a sibling

Diversity in Offspring There are 223 combinations for the way the homologous chromosomes could line up and separate This is more than 8 million combinations

Crossing Over In addition, crossing over in meiosis can increase diversity The chromosomes trade information, creating new combinations of information

Figure 6.6 Crossing over increases diversity in gametes. When chromosomes pair at the beginning of meiosis, information may be exchanged via the process of crossing over. (a) In our instruction manual analogy, this means that meiosis can result in the formation of completely new chromosome. (b) A photomicrograph of chromosomes in the process of crossing over during prophase of meiosis I.

Random Fertilization Gametes combine randomly—without regard to the alleles they carry in a process called random fertilization You are one out of 64 trillion genetically different children that your parents could produce

Diversity in Offspring Mutation, independent assortment, crossing over, and random fertilization result in unique combinations of alleles These processes produce the diversity of individuals found in humans and all other sexually reproducing biological populations

Twins Fraternal (non-identical) dizygotic: two separate fertilized eggs not genetically the same

Figure 6.7 The formation of twins. (a) Dizygotic twins form when 2 different eggs combine with 2 different sperm cells, resulting in 2 embryos who are only as similar as siblings.

Twins Identical monozygotic: one single fertilized egg that separates genetically the same

Figure 6.7 The formation of twins. (b) Monozygotic twins form when a newly fertilized embryo splits in half resulting in 2 identical embryos.

Mendelian Genetics: When the Role of Genes Is Clear

Gregor Mendel Determined how traits were inherited Used pea plants and analyzed traits of parents and offspring Figure E6.1a Gregor Mendel

Mendelian Genetics Mendelian genetics – the pattern of inheritance described by Mendel – for single genes with distinct alleles Sometimes inheritance is not so straightforward

Genotype Genotype – combination of alleles homozygous: two of the same allele heterozygous: two different alleles

Phenotype Phenotype the physical outcome of the genotype depends on nature of alleles

Mendelian Genetics Dominant – can mask a recessive allele Recessive – can be masked by a dominant allele Incomplete dominance – alleles produce an intermediate phenotype Codominance – both alleles are fully expressed

Mendelian Genetics Dominant alleles – capital letter For example: T for tall Recessive alleles – lower case letter For example: t for short

Figure 6.8 Genotypes and phenotypes. When 2 alleles for a gene exist in a population, there are 3 possible genotypes and 2 or 3 possible phenotypes for the trait.

Genetic Diseases in Humans Most alleles do not cause diseases in humans There are some diseases that are genetic: Recessive, such as cystic fibrosis Dominant, such as Huntington’s Disease Codominant, such as sickle-cell anemia

Genetic Diseases: Cystic Fibrosis Affects 1 in 2500 individuals in European populations Recessive condition: individuals have 2 copies of cystic fibrosis allele Carriers – have one cystic fibrosis allele but do not have cystic fibrosis; can pass along allele to children

Genetic Diseases: Cystic Fibrosis Produces nonfunctioning proteins Normal protein transports chloride ion in and out of cells in lungs Result – thick mucus layer that is difficult from lungs and interferes with absorption of nutrients in intestines

Huntington’s Disease Dominant condition Fatal condition Only one Huntington’s allele needed Produces abnormal protein that clumps up in cell nuclei – especially nerve cells in the brain

Sickle-Cell Anemia Codominant – both alleles are expressed One allele codes for normal hemoglobin and the other codes for sickle-cell hemoglobin Figure 6.9 The effect of the sickle-cell allele. The sickle-cell anemia allele causes red blood cells to look sickled as seen here, on the right. At left, a normal red blood cell appears round and pillow-like.

Sickle-Cell Anemia If you have two normal hemoglobin alleles, you do not have the disease If you have two sickle-cell hemoglobin alleles, you have sickle-cell disease If you have one of each, you are a carrier

Punnett Squares Punnett squares are used to predict offspring phenotypes Uses possible gametes from parents to predict possible offspring

Figure 6.10 What are the risks of accepting sperm from a carrier of cystic fibrosis? This Punnett square helps determine the likelihood that a woman who carries the cystic fibrosis allele would have a child with cystic fibrosis if her sperm donor was also a carrier. With a 25% chance of producing an affected child, most women would consider sperm from a carrier unacceptable, which is why sperm banks do not accept sperm from carrier males.

Punnett Squares: Single Gene A parent who is heterozygous for a trait Aa can produce two possible gametes A or a A parent who is homozygous for a trait AA can only produce gametes with A

Punnett Squares The possible gametes are listed along the top and side of the square The predicted offspring genotypes are filled in the center boxes of the square

Punnett Squares The offspring can be homozygous or heterozygous It all depends on the parents and the possible gametes Punnet squares can be used to predict possibilities of inheriting genetic diseases

Figure 6.11 Calculating the likelihood of genetic traits in children. (a) This Punnett square illustrates the outcome of a cross between a man who carries a single copy of the dominant Huntington’s disease allele and an unaffected woman.

Figure 6.11 Calculating the likelihood of genetic traits in children. (b) The outcome of a cross between two individuals with the sickle-cell trait. The two alleles are codominant—allele S codes for the normal blood protein, and allele s codes for the “sickling” blood protein.

Punnett Squares This is a probability for each individual offspring If there is a 25% chance an offspring will have cystic fibrosis – this means that – for every fertilization event, there is a 25% chance of cystic fibrosis

Punnett Squares: Multiple Genes You can also use Punnett squares to predict the offspring with multiple genes It is more significantly more difficult as the number of genes being studied increases

Figure 6.12 A dihybrid cross. Punnett squares can be generated to predict the outcome of a cross between individuals when we know their genotypes for more than one gene as long as those genes are on separate chromosomes. This square shows a cross between two pea plants that are heterozygous for both the seedcolor and the seed-shape genes.

Quantitative Genetics The environment plays a role – traits such as height, weight, musical ability, susceptibility to cancer, and intelligence Quantitative traits show continuous variation; we can see a large range of phenotypes in the population The amount of variation in a population is called variance

Figure 6.13 A quantitative trait. (a) This graph of the number of men in each category of height is a normal distribution with a center around the mean height of 1.73 m.

Figure 6.13 A quantitative trait. (b) Fourteen-year-old boys and professional jockeys have the same average weight—approximately 114 pounds. However, to be a jockey, you must be within about 4 pounds of this average. Thus the variance among jockeys in weight is much smaller than the variance among 14-year-olds.

Why Traits Are Quantitative Polygenic traits – those traits influence by more than one gene Eye color is a polygenic trait There are two genes: pigment and distribution This produces a range of eye colors

Why Traits Are Quantitative Environment can affect phenotypes Identical twins with the same genotypes may not have exactly the same appearance…

Figure 6.14 The effect of the environment on phenotype. These identical twins have exactly the same genotype, but they are quite different in appearance due to environmental factors.

Why Traits Are Quantitative Skin color is affected by both genes and environment…

Why Traits Are Quantitative Figure 6.15 Skin color is influenced by genes and environment. (a) The difference in skin color between these two women is due primarily to variations in several alleles that control skin pigment production. (b) The difference in color between the sun-protected and sun-exposed portions of the individual in this picture is entirely due to environmental effects.

Using Heritability to Analyze Inheritance Inheritance patterns for these quantitative traits are difficult to understand Researchers use plants and domestic animals to study heritability – a measure of the relative importance of genes in determining variation in quantitative traits among individuals

Using Heritability to Analyze Inheritance Artificial selection: controlling the reproduction of organisms to achieve desired offspring

Figure 6.16 Artificial selection increases milk production in cows. Cows that produce exceptional amounts of milk are bred to produce the next generation of dairy cattle. In this example, the female calves of the cow that produces 3.6 gallons of milk daily produce an average of 3.2 gallons of milk per day—23% more than the previous herd.

Calculating Heritability in Human Populations We can’t use artificial selection for humans So we look at correlations

Correlations between Parents and Children Inject parent and offspring in a bird population Look for correlation between parents and offspring in ability to produce anti-tetanus proteins

Figure 6.17 Using correlation between parents and offspring to calculate heritability. In this example, the close correlation of immune system response between parents and offspring of a bird species called the blue tit indicates that much of the variation among birds in the strength of their immune response is due to variation in their genes.

Correlations between Parents and Children For human IQ, the correlation between parents and offspring is 0.42 There is also an effect of society and environment on IQ Nature versus nurture debate – “born that way” or because they were “raised that way”

Correlation between Twins Twin studies allow scientists to test the effects of environment The DNA is identical in identical twins but the environment may be different Compare monozygotic (identical) twins to dizygotic (fraternal) twins Study twins raised together and study identical twins raised apart

Table 6.1 To what extent is IQ heritable? A summary of various estimates of IQ heritability, their shortcomings, and the problems with using them to understand the role of genes in determining an individual’s potential intelligence.

Genes, Environment, and the Individual

The Use and Misuse of Heritability Calculated heritability values are unique to a particular environment Therefore, we must be cautious when using heritability to measure the general importance of genes to the development of a trait

The Use and Misuse of Heritability The environment may cause large differences among individuals, even if a trait has high heritability Highly heritable traits can respond to environmental change Traits can be both highly heritable and strongly influenced by the environment

The Use and Misuse of Heritability Knowing the heritability of a trait does not tell us why two individuals differ for that trait Our current understanding of the relationship between genes and complex traits does not allow us to predict the phenotype of a particular offspring from the phenotype of its parents