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Chapter 7: Genes and Inheritance Family resemblance: how traits are inherited Lectures by Mark Manteuffel, St. Louis Community College 7.1 Family resemblance: your mother and father contribute equally to your genetic makeup. How can a single bad gene make you smell like a rotten fish? Take-home message 7.1 Offspring resemble their parents because they inherit genes from their parents. Genes are instruction sets for biochemical, physical, and behavioral traits. 7.2 Some traits are controlled by a single gene. Heredity • the passing of characteristics from parent to offspring through their genes Selective Breeding: Observing Heredity Single-Gene Traits Some traits are determined by the instructions an organism carries on one gene. Are there multiple-gene traits too? • Yes • Let’s first examine the mechanism by which single-gene traits pass from parent to child • We’ll then expand this model of heritability Take-home message 7.2 • Some traits are determined by instructions that an individual carries on a single gene, and these traits exhibit straightforward patterns of inheritance. 7.3 Mendel learned about heredity by conducting experiments. Take-home message 7.3 • In the mid-1800s, Gregor Mendel conducted studies that help us understand heredity. • He focused on easily observed and categorized traits in garden peas and applied methodical experimentation and rigorous hypothesis testing to determine how traits are inherited. 7.4 Segregation: you’ve got two copies of each gene but put only one copy in each sperm or egg. A dominant trait masks the effect of a recessive trait. Three Ideas Mendel Used for Explaining This Pattern of Inheritance 1) Each parent puts into every sperm or egg it makes a single set of instructions for building the trait. Three Ideas Mendel Used for Explaining This Pattern of Inheritance 2) Offspring receive two copies of the instructions for any trait (called alleles). Three Ideas Mendel Used for Explaining This Pattern of Inheritance 3) The trait observed in an individual depends on the two copies of the gene it inherits from its parents. • homozygous and heterozygous Take-home message 7.4 • Each parent puts a single set of instructions for building a particular trait into every sperm or egg he or she makes. • This instruction set is called a gene. • The trait observed in an individual depends on the two copies (alleles) of the gene it inherits from its parents. 7.5 Observing an individual’s phenotype is not sufficient for determining its genotype. Phenotypes and Genotypes The outward appearance of an individual is called their phenotype. Underlying genotype. the phenotype is the • This is an organism’s genetic composition. Genotypes Homozygous dominant Heterozygous How do we analyze and predict the outcome of crosses? Assign symbols to represent the different variants of a gene. Generally we use an uppercase letter for the dominant allele and lowercase for the recessive allele. Take-home message 7.5 • It is not always possible to determine an individual’s genetic makeup, known as its genotype, by observation of the organism’s outward appearance, known as its phenotype. Take-home message 7.5 • For a particular trait, an individual may carry a recessive allele whose phenotypic effect is masked by the presence of a dominant allele. • Much genetic analysis makes use of clever experiments and careful recordkeeping, often using Punnett squares, to determine organisms’ genotypes. Probability and chance play central roles in genetics. 7.6 Chance is important in genetics. Probability has a central role in genetics for two reasons: The The first is a consequence of segregation. second reason is that fertilization, too, is a chance event. Probabilities Any gamete produced by an individual heterozygous for a trait has a 50% probability of carrying the dominant allele and a 50% probability of carrying the recessive allele. Probabilities a male is heterozygous for albinism (Mm) and a female is homozygous for albinism (mm), what is the probability that their child will be homozygous for albinism (mm)? If Take-home message 7.6 • Probability plays a central role in genetics. • In segregation, each gamete that an individual produces receives only one of the two copies of each gene the individual carries • It is impossible to know which allele goes into the gamete. Take-home message 7.6 • Chance plays a role in fertilization, too: all of the sperm or eggs produced by an individual are different from one another, and any one of those gametes may be the gamete involved in fertilization. 7.7 A test-cross enables us to figure out which alleles an individual carries. You would like to produce white alligators via a mating program. The problem is that you cannot be certain of the genotype of your alligators. They might be homozygous dominant, MM, or they might be heterozygous, Mm. In either case their phenotype is normal coloration. How can you figure out which of these two possibilities is the actual genotype? Take-home message 7.7 • In a test-cross, an individual that exhibits a dominant trait but has an unknown genotype is mated with an individual that is homozygous recessive. Take-home message 7.7 • The phenotypes of the offspring reveal whether the unknown-genotype individual is homozygous dominant (all of the offspring exhibit the dominant trait) or heterozygous (half of the offspring show the dominant trait and half show the recessive trait). 7.8 Using pedigrees to decipher and predict the inheritance patterns of genes. Pedigree: a type of family tree Analyzing Which Individuals Manifest the Trait and Which Do Not Sex-Linked Traits A Trait’s Mode of Inheritance Is Not Always Completely Obvious Some traits may not show complete dominance. Many traits are influenced by the environment Take-home message 7.8 Pedigrees help scientists, doctors, animal and plant breeders, and prospective parents determine: • the genes that individuals carry • the likelihood that the offspring of two individuals will exhibit a given trait. How are genotypes translated into phenotypes? 7.9 Incomplete dominance and codominance: the effects of both alleles in a genotype can show up in the phenotype. Incomplete dominance, in which the heterozygote appears to be intermediate between the two homozygotes. Codominance, in which the heterozygote displays characteristics of both homozygotes. Take-home message 7.9 • Sometimes the effects of both alleles in a heterozygous genotype are evident in the phenotype. Take-home message 7.9 • With incomplete dominance, the phenotype of a heterozygote appears to be an intermediate blend of the phenotypes of the two homozygotes. • With codominance, a heterozygote has a phenotype that exhibits characteristics of both homozygotes. 7.10 What’s your blood type? Some genes may have more than two alleles. Multiple Allelism in which a single gene has more than two alleles each individual still carries only two alleles Inheritance of the ABO Blood Groups A, B, and O alleles The A and B alleles are both completely dominant to O. The A and B alleles are codominant to each other. Individuals can be one of four different blood types: A, B, AB, and O. Why are people with type O blood considered “universal donors”? Why are those with type AB considered “universal acceptors”? Take-home message 7.10 In multiple allelism, a single gene has more than two alleles. Each individual still only carries two alleles, but more alleles occur in the population. This is the case for the ABO blood groups in humans. 7.11 Multi-gene Traits How are continuously varying traits such as height influenced by genes? Polygenic Trait A trait that is influenced by many different genes Additive Effects what happens when the effects of alleles from multiple genes all contribute to the ultimate phenotype Take-home message 7.11 Many traits, including continuously varying traits such as height, eye color, and skin color, are influenced by multiple genes. 7.12 Sometimes one gene influences multiple traits What is the benefit of “almost” having sickle cell disease? The SRY Gene “Sex-determining chromosome” Region on the Y- Causes fetal gonads to develop as testes shortly after fertilization. Following the gonads’ secretion of testosterone, other developmental changes also occur. Take-home message 7.12 In pleiotropy, one gene influences multiple unrelated traits. Most, if not all, genes may be pleiotropic. 7.13 Why are more men than women color-blind? Sex-linked traits differ in their patterns of expression in males and females. If a man is color-blind, did he inherit this condition from his mother, his father, or both parents? Take-home message 7.13 The patterns of inheritance of most traits do not differ between males and females. When a trait is coded for by a gene on a sex chromosome, such as color vision on the X chromosome, the effects differ in males and females. 7.14 THIS IS HOW WE DO IT What is the cause of male-pattern baldness? Can you propose how to test the “hats cause baldness” hypothesis? Are these hypotheses falsifiable? Propose how to test each of them. “The hair follicles are clogged from shampoo and too much washing.” “Not enough blood is circulating around the scalp.” “Hair gels and other products are toxic.” A Modern Approach Balding men commonly shared a DNA sequence on their X chromosome. What does that tell us about whom they inherited that DNA from? Stopping short of the one-gene = one trait conclusion. Take-home message 7.14 • Observations of male-pattern baldness within families and comparisons with unrelated individuals suggest that the baldness is caused by a sex-linked gene that codes for an androgen receptor. Take-home message 7.14 • Males inheriting an allele—always from their mother—for higher activity of the androgen receptor gene are more likely to have male-pattern baldness than males inheriting an alternative allele. 7.15 Environmental effects: identical twins are not identical. Drinking diet soda can be deadly if you carry a single bad gene. What gene is it and why is it so deadly? Could you create a temporarily spotted Siamese cat with an ice pack? Why? Take-home message 7.15 Genotypes are not like blueprints that specify phenotypes. Phenotypes are generally a product of the genotype in combination with the environment. Some genes are linked together. 7.16 Most traits are passed on as independent features: Mendel’s law of independent assortment. Take-home message 7.16 Genes The tend to behave independently. inheritance pattern of one trait doesn’t usually influence the inheritance of any other trait. 7.17 Red Hair and Freckles Genes on the same chromosome are sometimes inherited together. Why do most redheads have pale skin? Linked genes: Genes on the same chromosome, maybe even right next to each other. Take-home message 7.17 Sometimes, having one trait influences the presence of another trait. This is because the alleles for two genes are inherited and expressed together almost as a package deal when the genes are located close together on the same chromosome.