Lab. 9 Deviation of Mendel s first law Monohybrid part 2

Transcription

1 Main topics: Lab. 9 Deviation of Mendel s first law Monohybrid part 2 Deviation of Mutation a. ABO type b. Fur color of rabbits Deviation of Sex a. Sex limited b. Sex influence c. Sex linkage Deviation from Mendel s law Sometimes during the crosses the ratio obtained does not obey Mendelian s law. Deviation from Mendel s law I: 1. Dominance: a. Co-dominance b. Incomplete dominance 2. Lethal alleles c. Dominance lethal alleles d. Recessive lethal alleles 3. Mutation: a. Multiple alleles 4. Sex : a. Sex- limited b. Sex- influences c. Sex- linkage Principles of Genetics 1

2 3. Multiple alleles: In Mendel's studies, he proposed that there are two alleles for every gene, the dominant of the two having its phenotype expressed in a heterozygote. However, a gene can have more than two allelic forms segregating within a population. These genes are referred to as having multiple alleles. This does not mean that the gene in a particular individual possesses more than two alleles. An individual can only have a maximum of two of the alleles, one maternal and one paternal, no matter how many alleles exists in the population. Examples: ABO blood types are an example of multiple alleles. Fur color in rabbits is determined by one gene with four different possible alleles. 1. ABO blood types For the ABO gene, the three alleles are the I A, I B and i alleles. We typically call these alleles "A," "B," and "O," but of course our rules for assigning symbols to alleles demand that all three be represented by some version of the same symbol. In this case, that common symbol is the letter "I," which stands for Isoagglutinogen." By now, the concept of dominance should be familiar to you. Of course, things get a bit more complicated when there are three alleles instead of just two. As the symbols above should suggest, the i allele (the "O" allele) is recessive to both the I A and I B alleles (the "A" and "B" alleles). The I A and I B show Co-Dominance. This means that in an individual who is heterozygous for these two alleles, the phenotypes of both alleles are completely expressed, thus producing blood type AB. Principles of Genetics 2

3 Thus we have the following: Genotype ii Phenotype Type O I A I A or I A i Type A I B I B or I B i Type B I A I B Type AB ABO blood group antigens present on red blood cells and IgM antibodies present in the serum: Principles of Genetics 3

4 Genetic Analysis of Blood group: P: Gp: F1: 2. Fur color in rabbits is determined by one gene with four different possible alleles. Fur color in rabbits is determined by a single gene loci for which there are four alleles. Four phenotypes are possible: Black, Chinchilla (gray color caused by white hairs with black tips), Himalayan (white with black patches on extremities of rabbit), and white. The black allele (C) is dominant over all other alleles, Chinchilla (C ch ) is dominant over Himalayan (C h ), and white or Albino (c) is recessive to all others. Figure: Fur color in rabbits Principles of Genetics 4

5 Ex. Fur color in rabbits. What is the genotype and the Phenotype of the parents and offspring in hybrids following: Wild x wild ¾ wild : ¼ Himalayan Genetic Analysis: P: Gp: F1: Wild x Himalayan ½ wild : ¼ Himalayan : ¼ Albino Genetic Analysis: P: Gp: F1: Gender & inheritance: For most inherited traits, the gender of the bearer of the genes is immaterial. Characteristics like free earlobes, fur color, etc., generally operate the same in males as they do in females. But there are exceptions. These fall into three primary categories. Sex limited traits There are many examples where the action of a gene is influenced by the sex of the individual. This is known as sex-limited inheritance (also known as sex-controlled inheritance) and, in Butterflies, typically affects color, wing shape, markings and behavior. The resulting sexual dimorphism results from the action of sex-limited genes which, although present in both sexes, only have an effect in one sex. We have two examples in the British Isles - the valesina form of the female Silver-washed Principles of Genetics 5

6 Fritillary and the helices form of the female Clouded Yellow, where the named form is only expressed in the female. In both cases, the allele controlling the form is dominant (which has been proven through captive rearing and examining the ratios of offspring) rather than recessive (which is what we might expect given the ratios we see in the wild, which we'll come back to). Now let's consider a couple of pairings. The figure below shows a pairing of a normal male (that doesn't carry the valesina allele) with a valesina female. In this example, we're considering both the sex chromosomes (X and Y), as well as the allele for wing color (represented using the letter V). Since the male is XXvv, the only combination of alleles in the gametes is Xv (as shown). We can see that we get equal numbers of normal and valesina females in the offspring. Figure: Sex limited Principles of Genetics 6

7 Sex influenced traits Sex influenced traits are also autosomal, meaning that their genes are not carried on the sex chromosomes. Again, what makes these traits unusual is the way they are expressed phenotypically. In this case, the difference is in the ways the two genders express the genes. Figure: Sex influenced One classic example of a sex influenced trait is Pattern Baldness in humans (sometimes called male pattern baldness, though the condition isn t restricted to males). This gene has two alleles, bald and non-bald. The behaviors of the products of these genes are highly influenced by the hormones in the individual, particularly by the hormone testosterone. In the presence of high levels of testosterone, the baldness allele has a very powerful influence. In the presence of low levels of testosterone, this allele is quite ineffectual. All humans have testosterone, but males have much higher levels of this hormone than females do. The result is that in males, the baldness allele behaves like a dominant allele, while in females it behaves like a recessive allele. As in all cases, dominance only matters in the heterozygote, so this means that heterozygous males will experience hair loss and heterozygous females will not. Even homozygous females may experience no more than a thinning of their hair, but many develop bald spots or have receding hairlines. Ex. Baldness in human Genetics Analysis: P: HH x hh ( hair, baldness) x ( baldness, hair) F1: Hh ( hair less, baldness) F2: HH : 2 Hh: hh Principles of Genetics 7

8 Sex Linked Traits: Sex Linked Traits are traits whose loci are literally on the sex chromosomes, so their transmission from generation to generation is affected by the sex chromosome complement of the individual. In any species with non-homologous sex chromosomes, these traits can be significant. The first demonstration of sex linkage was the white eye gene in Drosophila, the fruit fly which has become so important to the study of classical genetics. Normal fruit fly eye color is a dull brick red. Mutations in this gene cause the eyes to be white. The white allele is recessive, but it was quickly determined that the inheritance pattern for this gene was different from those of other genes being studied. In some kinds of mating, reciprocal crosses produced different results, something which had never been observed to happen with other genes. Not only that, but in some kinds of mating, the results for the male offspring would be different from the results for the female offspring. For instance, if a white-eyed female was mated to a red-eyed male, all of the female offspring would be red eyed, but all of the male offspring would be white eyed. It turned out that this particular eye color gene was literally located on the X chromosome. Since females have two X chromosomes and males have only one, genetic effects in the two genders are different. And since females contribute an X to each offspring, male or female, and males contribute X chromosomes only to female offspring, naturally transmission patterns were different in different kinds of mating. Linkage refers to the physical connection that exists between genes whose loci are on the same chromosome. Genes on the X chromosome are all linked to each other-thus they are X-linked. Principles of Genetics 8

9 Figure: sex linked in Drosophila melanogaster. Ex. Sex linked in Drosophila melanogaster ( wild, white) Genetics P: Analysis: F1: F2: Ex. Sex linked in Drosophila melanogaster ( white, wild) P: F1: F2: Principles of Genetics 9

10 Gender Human Roach Butterfly Chicken Male XY XO XX XX Female XX XX XY XO References: Yazer M, Olsson M, Palcic M; Olsson; Palcic (2006). "The cis-ab blood group phenotype: fundamental lessons in glycobiology". Transfus Med Rev 20 (3): doi: /j.tmrv PMID Muramatsu M, Gonzalez HD, Cacciola R, Aikawa A, Yaqoob MM, Puliatti C (2014)."ABO incompatible renal transplants: Good or bad?". WORLD JOURNAL OF TRANSPLANTATION 4 (1): doi: /wjt.v4.i1.18. PMID Principles of Genetics 10

11 Exercise 9 1. Choose the right answer from the following: A child is born with the blood type B. The mother of the child is blood type O. Which of the following statements is most correct? (A) The father must be type B (B) The father could be type A (C) The father passed along a recessive allele (D) The mother could have passed along a type B allele (E) The mother could have passed along a dominant allele Hemophilia is a sex-linked recessive trait. A male hemophiliac and phenotypically normal female have a girl that is a hemophiliac. All of the following statements are correct EXCEPT: (A) The daughter inherited a recessive gene from each parent. (B) The daughter inherited a dominant allele from her mother. (C) The mother is a carrier of hemophilia (D) The genotype of the mother is X H X h (E) The genotype of the father is X h Y Suppose a newborn baby was accidentally mixed up in the hospital. In an effort to determine the parents of the baby, the blood types of the baby and two sets of parents were determined. Baby 1 had type O Mrs. Brown had type B Mr. Brown had type AB. Mrs. Smith had type B Mr. Smith had type B. a. Draw Punnett squares for each couple (you may need to do more than 1 square/couple). b. To which parents does baby #1 belong? Why? Hint you may want to refer to your Punnett squares. Principles of Genetics 11

12 A man heterozygous for blood type B marries a woman heterozygous for blood type A. What is the chance that their first offspring will have type O? Hemophilia in humans is inherited as an X linked recessive trait. A woman whose father is hemophilia marries a man with normal clotting ability. What is the probability that her first child will have hemophilia? Assume that the woman's mother is homozygous dominant. Red/Green color blindness is a sex linked traits. If a man with normal color vision marries a colorblind woman, will all of the offspring be colorblind? If not, who won't be colorblind (sons or daughter)? Principles of Genetics 12

13 What is the probability of child will be bald if his father is homozygous and balding, and his mother is homozygous and not balding? A male homozygous for clubfoot reproduces with a normal homozygous female. What are the genotypes and phenotypes of their children if testosterone alters the phenotypic expression of the trait in the heterozygous expression? 1. Let F represent normal feet 2. Let f represent clubfoot. Two heterozygous individuals get married and have lots of children. The father suffers with a painful condition called gout. What are the parent s genotypes? What are the genotypes and phenotypes of the children? Let G represent no gout, and g represent gout. Principles of Genetics 13