Lecture 4: Inheritance Patterns PDF

Chapters 9 and 10 1 Humans Have Used the Principles of Inheritance for Thousands of Years to Domesticate Plants and Animals Shar-pei, one of the oldest dog breeds The field of genetics originated in 1866 aft...

Chapters 9 and 10 1 Humans Have Used the Principles of Inheritance for Thousands of Years to Domesticate Plants and Animals Shar-pei, one of the oldest dog breeds The field of genetics originated in 1866 after Gregor Mendel published a paper on inheritance in pea plants. Mendel’s work was largely ignored for 30 years before it was adopted as the foundation for modern genetics. Genetics is the study of inherited characteristics (genetic traits) and the genes Gregor Mendel (1822–1884) 2 that affect those traits.  A Genetic Trait Is Any Inherited Characteristic of an Organism That Can Be Observed or Detected Invariant genetic traits are the same in all individuals in a population. Variant genetic traits come in two or more different versions, or phenotypes. The display of a particular version of a genetic trait in a specific individual is the phenotype of that genetic trait in that individual. 3  Some Phenotypes Are Controlled by Dominant Alleles Phenotype: the observed expression of a trait or traits Genotype: the combination of alleles that determines the phenotype The allele that exerts a controlling influence on the phenotype in a heterozygote is said to be dominant. An allele that has no effect on the phenotype when paired with a dominant allele in a heterozygote is said to be recessive. 4  Recessive does not mean “bad” or “rare” Mackerel tabby caused by Classic tabby caused by dominant Mc allele. recessive mc allele. Recessive simply means that two identical alleles are required to have that phenotype. 5  Breeding Trials Help Us Understand Patterns of Inheritance A genetic cross is a controlled mating experiment performed to examine how a particular trait is inherited. The parents, or P generation, are crossed to produce offspring, called the F1 generation. Two individuals from the F1 generation are then crossed to produce the F2 generation. 6 Mendel Began By Studying the Inheritance of Single Traits In a single-trait cross, the experimenter tracks the inheritance of the two alleles of a single gene. If all F2 offspring are hybrids for that one trait, as they were in all of Mendel’s experiments, this type of cross is a monohybrid cross. For the P generation: Mendel crossed true-breeding pea plants with contrasting phenotypes for a particular genetic trait, such as flower color. He performed many such crosses and recorded the phenotypes of the resulting F1 generation; next he crossed individuals of the F1 generation to raise the F2 7 generation.  Mendel Observed a 3:1 Ratio of Dominant to Recessive Phenotypes in the F2 Generation Mendel’s observations: Odds that the dominant phenotype will be seen in the F1 generation: 100 percent (4 in 4) Odds that the recessive phenotype will reappear in the F2 generation: 1 in 4 (25 percent) Phenotypic ratio in F1 (dominant to recessive phenotype) is 3:1 Genotypic ratios in F2: 1:4 (25 percent) PP 1:2 (50 percent) Pp 1:4 (25 percent) pp 8  Mendel’s Experiments with Two Traits (Dihybrid Crosses) Next, Mendel sought to determine if a particular phenotype of one trait is always inherited together with a particular phenotype of a different trait. Is the yellow seed color always inherited with the round seed shape? Or would he find combinations of phenotypes (yellow color and wrinkled shape) among the offspring that were not present in the P generation? Mendel’s question was rather like asking whether a particular eye color phenotype (say, blue eyes) always goes with a particular hair color phenotype (say, blond hair). 9  A Punnett Square Can Be Used to Show All the Possible Ways in Which Two Alleles Can Recombine Through Fertilization 10  A summary of Mendel’s 5 discoveries: 1. Parents do not transmit traits directly to offspring, rather they transmit information about traits (allelic versions of genes) 2. Each parent contains 2 copies of the genes governing each trait If each copy (allele) is the same, it is homozygous (ex. BB or bb) If the two copies (alleles) are different, it is heterozygous (ex. Bb) 3. The two alleles that an individual possesses do not affect each other 4. The presence of an allele does not ensure that a trait will be expressed in the individual that carries it 5. Alternative forms of a gene (= alleles) lead to alternative traits 11  The discoveries led to Mendel’s Laws Law of Segregation: During the production of gametes (eggs or sperm), the two copies of each gene (allele) segregate so that offspring acquire one gene from each parent. Law of Independent Assortment: When two or more characteristics (genes) are inherited, individual heredity factors assort independently during gamete production, giving different traits (alleles) an equal opportunity of occurring together. 12  Non-Mendelian inheritance Incomplete dominance: an intermediate, heterozygous phenotype Red (CR) and White (CW) 13  Non-Mendelian inheritance Co-dominance: the effects of both alleles are expressed; no dominant allele – Parts show phenotype of allele 1; parts show phenotype of allele 2 It is different from incomplete dominance – Incomplete dominance blends the two alleles (red + white = pink) 14 The inheritance of blood types in humans are an example of co-dominance: Blood types: A, B, AB, O come from 3 different alleles The alleles put different sugar molecules onto the surface of blood cells for cell identification 15  Non-Mendalian Inheritance The situation in which a single gene influences two or more distinctly different traits is called pleiotropy. A mutation in a pleiotropic gene can cause changes in many different traits. Albinism is an example of a pleiotropic disorder. Albinism is caused by a single recessive allele affecting pigment formation, but other traits such as vision are also 16 affected.  Non-Mendalian Inheritance Alleles for One Gene Can Alter the Effects of Another Gene The term epistasis applies when the phenotypic effect of the alleles of one gene depends on the presence of certain alleles for another, independently inherited gene. Epistasis can be seen in the coat color of numerous animals, whose many genes code 17 for enzymes that convert the amino acid tyrosine into melanin in a multistep pathway.  Non-Mendalian Inheritance Environmental variation: The environment can alter the phenotype Chemicals, nutrition, sunlight, and other internal and external environmental factors can alter the effects of certain genes. The production of melanin in Siamese cats is sensitive to temperature— cooler temperatures produce dark fur on the extremities. 18  Non-Mendalian Inheritance Traits governed by the action of more than one gene are polygenic traits. These traits lead to a range of phenotypes being expressed in a population. Skin color, running speed, blood pressure, and body size are all polygenic traits in humans. Skin color in humans, and many other mammals, is controlled by multiple genes. Example: Assuming control by three genes, each with two incompletely dominant alleles, seven different phenotypes are possible in the offspring of triple- heterozygote parents. 19  Polygenic Traits, Combined with Environmental Influences, Produce a Smoothly Graded Range of Phenotypic Classes or Continuous Variation Now, consider the influence of sun exposure on the seven Predicted phenotypes. An even broader range of phenotypes, from very pale to very dark, is possible. Geneticists estimate there are more than a dozen genes that control melanin production in our skin, which, when coupled with environmental influences, results in continuous variation in the trait. 20  Most Traits That Are Essential for Survival Are Complex Traits Complex traits are those that cannot be predicted using Mendel’s laws of inheritance; complex traits display often display continuous variation in a population. According to one hypothesis, the evolutionary benefit of continuous variation in phenotypes is that if the environment changes, there are good odds that one out of the many phenotypes will be adaptive under the new conditions. 21  In Humans, Maleness Is Specified by the Y Chromosome In mammals, female gametes all contain an X chromosome; male gametes contain either an X or a Y chromosome. The sex chromosome carried by the sperm determines the sex of the offspring. The Y chromosome carries the SRY gene, a master gene that causes other genes located on autosomes to produce male sexual characteristics; without the SRY gene, the embryo develops as a female. 22  Autosomes Differ from Sex Chromosomes Chromosomes that determine sex are called sex chromosomes; all other chromosomes are called autosomes. In humans, males have one X chromosome and one Y chromosome and females have two X chromosomes. Human males have only one copy of each gene that is unique to either the X or the Y chromosome. 23  Some “practice problems” Chapter 9 Multiple Choice: 1-2, 4-7 Sample Genetics Problems: 2, 3-4, 5 (a. and b. only), 6-10 Note: we will do a lot more with Punnett Squares in lab. You will need to be able to do Monohybrid Crosses for the lecture exam, but not Dihybrid Crosses. 24