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10 Genetics: Mendel and Beyond
10 By the end of this chapter you should be able to: Describe Mendel’s Experiments and the Laws of Inheritance Predict inheritance patterns of monohybrid and dihybrid crosses using a punnett square Explain examples of gene interactions Distinguish between genes and chromosomes Explain sex determination and Predict sex-linked inheritance
10 The Foundations of Genetics Applied genetics (plant and animal breeding) has been used for five thousand years ago or more. The foundation for the science of genetics is credited to Gregor Mendel who used varieties of peas to conduct experiments on inheritance. Mendel’s research was ignored until the turn of the twentieth century. Meiosis provides an explanation for Mendel’s theory.
10 The Foundations of Genetics Plants have some desirable characteristics for genetic studies: They can be grown in large quantities. They produce large numbers of offspring. They have relatively short generation times. Many have both male and female reproductive organs, making self-fertilization possible. It is easy to control which individuals mate.
Figure 10.1 A Controlled Cross between Two Plants
10 Mendel’s Experiments and the Laws of Inheritance Mendel looked for characters that had well- defined alternative traits and that were true- breeding. Mendel developed true-breeding strains to be used as the parental generation, designated P. The offspring from the cross of the P parents are called the first filial generation, designated F 1. When F1 individuals are crossed to each other or self-fertilized, their progeny are designated F 2.
10 Mendel’s Experiments and the Laws of Inheritance Mendel’s experiment 1: A monohybrid cross involves one character (seed shape) and different traits (spherical or wrinkled). SS x ss → Ss The F1 seeds were all spherical; the wrinkled trait failed to appear at all. Because the spherical trait completely masks the wrinkled trait when true-breeding plants are crossed, the spherical trait is considered dominant and the wrinkled trait recessive. We represent the dominant trait with a capital letter and the recessive trait with a small case letter.
10 Mendel’s Experiments and the Laws of Inheritance Mendel’s experiment 1 continued: The F 1 generation was allowed to self-pollinate to produce F 2 seeds. Ss x Ss → 3Ss, 1 ss 3 are heterozyous smooth and 1 is homozygous wrinked. In the F2 generation, the ratio of spherical seeds to wrinkled seeds was 3:1.
Figure Mendel’s Experiment 1 (Part 1)
Figure Mendel’s Experiment 1 (Part 2)
10 Mendel’s Experiments and the Laws of Inheritance From these results, Mendel reached several conclusions: The units responsible for inheritance are discrete particles that exist in pairs and separate during gamete formation; this is called the particulate theory. Each pea has two units of inheritance for each character. During production of gametes, only one of the pair for a given character passes to the gamete. When fertilization occurs, the zygote gets one unit from each parent, restoring the pair.
10 Mendel’s Experiments and the Laws of Inheritance Mendel’s units of inheritance are called genes (a portion of the chromosomal DNA that resides at a specific locus and codes for a particular function); different forms of a gene are called alleles. True-breeding individuals have two copies of the same allele (i.e., they are homozygous). Some smooth-seeded plants are Ss or heterozygous. The physical appearance of an organism is its phenotype (what it looks like); the actual composition of the organism’s alleles for a gene is its genotype. Homozygous dominant; heterozygous dominant; homozygous recessive
10 An organism’s trait does not always reveal its genetic composition. Why? Heterozygous genotypes yield phenotypes showing the dominant trait. The same phenotype can result from different genotypes.
10 Mendel’s Experiments and the Laws of Inheritance Mendel’s first law is called the law of segregation: The two alleles of a trait segregate (separate) during meiosis. Each gamete receives one member of a pair of alleles. Determination of possible allelic combinations can be accomplished by a Punnett square.
10 Punnett Square Another way to demonstrate this is through the use of a punnett square S S S s s Ss Ss S SS Ss s Ss Ss s Ss ss Since one character with two contrasting traits are being crossed, this is called a monohybrid cross
Figure 10.4 Mendel’s Explanation of Experiment 1
Figure 10.5 Meiosis Accounts for the Segregation of Alleles (Part 1)
Figure 10.5 Meiosis Accounts for the Segregation of Alleles (Part 2)
10 Mendel’s Experiments and Laws of Inheritance How do we determine if a purple flowering plant is SS or Ss? We could cross the purple flowering plant of unknown genotype with a true breeding purple flowering plant or a white flowering plant SS x SS (unknown) or SS x Ss (unknown) all spherical ss x SS (unknown) or ss x Ss (unknown) all spherical2 spherical: 2 wrinkled
10 Mendel’s Experiments and the Laws of Inheritance Crossing an unknown with a homozygous recessive is called a test cross. An individual with a dominant trait is crossed with a true-breeding recessive (homozygous recessive). The appearance of the recessive phenotype in half the offspring indicates that the parent is heterozygous.
Figure 10.6 Homozygous or Heterozygous?
10 Mendel’s Experiments and the Laws of Inheritance Mendel’s second law, the law of independent assortment, states that alleles of different genes assort into gametes independently. This can be shown by using a dihybrid crosses. For example, in pea plants purple flowers are dominant over white flowers and green pods are dominant over yellow pods. Cross a purebreeding purple flowering plant with green pods with a white flowering plant with yellow pods. Random fertilization of gametes results in all heterozygous offspring.
10 Dihybrid Crosses PPGG x ppgg = PpGg If we allow these plants to self-pollinate, PpGg x PpGg, what are the possible offspring? Each parent could produce four different gametes – PG, Pg, pG, or pg
10 Dihybrid Crosses PG Pg pG pg PG Pg pG pg
10 Dihybrid Crosses PG Pg pG pg PG PPGG PPGg PpGG PpGg Pg PPGg PPgg PpGg Ppgg pG PpGG PpGg ppGG ppGg pg PpGg Ppgg ppGg ppgg Putting these into a punnett square results in a 9:3:3:1 ratio
Figure 10.7 Independent Assortment
10 Mendel’s Experiments and the Laws of Inheritance Humans cannot be studied using planned crosses. Therefore, human geneticists rely on pedigrees. Human pedigrees do not show clear proportions. Outcomes for small samples fail to follow the expected outcomes closely.
10 Mendel’s Experiments and the Laws of Inheritance If neither parent has a given phenotype, but it shows up in their offspring, the trait is recessive and the parents are heterozygous. Half of the children from such a cross will be carriers (heterozygous for the trait). The chance of any one child’s getting the trait is 1/4.
Figure Recessive Inheritance
10 Mendel’s Experiments and the Laws of Inheritance A pedigree analysis of the dominant allele for Huntington’s disease shows that: Every affected person has an affected parent. About half of the offspring of an affected person are also affected (assuming only one parent is affected). The phenotype occurs equally in both sexes.
Figure Pedigree Analysis and Dominant Inheritance
10 Figure figure jpg
10 Huntington’s Disease
10 Alleles and Their Interactions Differences in alleles of genes consist of slight differences in the DNA sequence at the same locus, resulting in slightly different protein products. This is a mutation. Alleles can mutate randomly.
10 Alleles and Their Interactions A population can have more than two alleles for a given gene. In rabbits, coat color is determined by one gene with four different alleles. Five different colors result from the combinations of these alleles. Even if more than two alleles exist in a population, any individual can have no more than two of them: one from the mother and one from the father. Also send in ABO blood typing.
Figure Inheritance of Coat Color in Rabbits
10 Alleles and Their Interactions A white snapdragon crossed with a red snapdragon, gives an intermediate phenotype – pink Incomplete Dominance In the case of snapdragons, one allele codes for an enzyme that leads to the formation of red pigment. The other allele does not code for pigment production.
10 The F 2 offspring, however, demonstrate Mendelian genetics. For self-fertilizing F1 pink flowers the F2 progeny have a phenotypic ratio of 1 red:2 pink:1 white. Other examples include roan cattle and blue Andalusian chickens Tay Sachs Disease is also an example of incomplete dominance.
10 Incomplete Dominance Persons with Tay Sachs lack a crucial enzyme to metabolize a type of lipid. The lipids accumulate in the brain interfering with normal function. Causes regression of nervous system, - blind, deaf, bedridden, inability to move limbs Cherry red spot in retina is one indication Heterozygotes have an intermediate level of the lipid metabolizing enzyme
10 Tay Sachs
10 Alleles and Their Interactions In codominance, the two different alleles are both expressed in the heterozygotes. In the human ABO blood group system the alleles for blood type are I A, I B, and I O. We inherit two of these three alleles. Two I A, or I A and I O, results in type A. Two I B, or I B and I O, results in type B. Two I O results in type O. I A and I B results in type AB. The alleles are called codominant.
Figure ABO Blood Reactions Are Important in Transfusions
10 ABO Genetic Problems A couple have their blood typed before marriage. They both are AB. What types of blood might their children have? Explain A woman sues a man for child support. She has type A blood, her child type O, and the man type B. Could the man be the father? Why or why not?
10 ABO Genetic Problems A wealthy, elderly couple die together in a car accident. Soon a young man shows up to claim their fortune, contending that he is their only son who ran away from home when he was a young man. Other relative dispute his claim. Hospital records show that the deceased couple were blood types AB and O. The claimant is type O. Do you think the claimant is an impostor? Explain.
10 Pleiotropic alleles Most genes have multiple phenotypic effects. An example is the coloration pattern and crossed eyes of Siamese cats, which are both caused by the same allele. These unrelated characters are caused by the same protein produced by the same allele. Another example is sickle cell anemia.
10 Sickle Cell Anemia
10 Gene Interactions In epistasis a gene at one locus alters the phenotypic expression of a gene at a second locus. An example is coat color in mice: The B allele produces a banded pattern, called agouti. The b results in unbanded hairs. The genotypes BB or Bb are agouti or banded. The genotype bb is black. Another locus determines if any coloration occurs. The genotypes AA and Aa have color and aa are albino. Cross two AaBb mice. What is the phenotype?
Figure Genes May Interact Epistatically
10 Gene Interactions When two homozygous strains of plants or animals are crossed, the offspring are often phenotypically stronger, larger, and more vigorous than either parent. This phenomenon is called hybrid vigor. Hybridization is now a common agricultural practice used to increase production in plants.
Figure Hybrid Vigor in Corn
10 The Environment Affects Gene Action Genotype and environment interact to determine the phenotype Environmental variables such as light, temperature, and nutrition can affect the translation of genotype into a phenotype Examples – Siamese cats, hydrangea flowers Twin Studies
10 Genes and Chromosomes Homologous chromosomes can exchange corresponding segments during prophase I of meiosis (crossing over). Genes that are close together tend to stay together. The farther apart on the same chromosome genes are, the more likely they are to separate during recombination.
Figure Crossing Over Results in Genetic Recombination
10 Sex Determination in Humans Sex chromosomes carry genes that determine whether male or female gametes are produced. In humans, the Y chromosome has a sex- determining region - SRY The SRY gene codes for a functional protein. If this protein is present, testes develop; if not, ovaries develop. Some XY individuals lacking a small portion of the Y chromosome are phenotypically female. Some XX individuals with a small piece of the Y chromosome are male.
10 Sex linked traits The Y chromosome carries few genes (20). The X chromosome carries many genes. This difference generates a special type of inheritance called sex-linked inheritance Two well-know traits carried on the X chromosome are colorblindness and hemophilia. Sex linked traits tend to be expressed with greater frequency in males. Can you explain why? This is due to the fact that many of these diseases are only on the X chromosome and not the Y
10 Figure Red-Green Color Blindness Is a Sex-Linked Trait in Humans
10 Sex Determination and Sex-Linked Inheritance Pedigree analysis of X-linked recessive phenotypes: The phenotype appears much more often in males than in females. A male with the mutation can pass it only to his daughters. Daughters who receive one mutant X are heterozygous carriers. The mutant phenotype can skip a generation if the mutation is passed from a male to his daughter and then to her son.
10 Sex Determination and Sex-Linked Inheritance Disorders can arise from abnormal sex chromosome constitutions. Turner syndrome is characterized by the XO condition and results in females who physically are slightly abnormal but mentally normal and usually sterile. The XXY condition, Klinefelter syndrome, results in males who are taller than average and always sterile.
10 Non-Nuclear Inheritance Mitochondria, chloroplasts, and other plastids possess a small amount of DNA. Some of these genes are important for organelle assembly and function. Mitochondria and plastids are passed on by the mother only, as the egg contains abundant cytoplasm and organelles. A cell is highly polyploid for organelle genes. Organelle genes tend to mutate at a faster rate.