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Transcript
Unit 8 Review Complete Dominance In some cats the trait for brown hair (B) is dominant to the trait for black (b) hair. Two brown cats are crossed. Most of the offspring have brown hair, and the rest of the offspring are black (bb). Create a Punnett square showing a cross of two brown cats that would result in some of the offspring being black. B b B BB Bb B Bb bb If hair color is completely dominant, the only way that two brown cats could have a brown kitten is if both of the parents were heterozygous for the trait. Certain kidney diseases are genetic and can be passed down from one generation to the next. In most cases these ate caused by a dominant allele. What is the chance that a child would have kidney disease if the father is unaffected and the mother is heterozygous for the disease? In order for the father to be unaffected he must be homozygous recessive (No copy of the dominant allele) B b b Bb bb b Bb bb Mom is heterozygous There would be a 50% chance that children of these parents would have kidney disease. But what happens if we know the phenotypes of the children, but knot of the parents? Can will still figure this out? Albinism for example is caused by a recessive allele. What would you expect the parents genotypes to be if they had 4 children, 3 that are normal and 1 that is albino. ? ? ? AA Aa ? aa Aa aa In order for one of the children to be albino, both of the parents must be heterozygous (Aa) {have at one copy of the albino allele} A a A AA Aa a aa Aa aa Test Crosses In many plants the allele for being tall (T) is dominant to the allele for being short (t). if you have a plant whose phenotype is tall (what we see) but do not know its genotype {remember it could be (TT) or (Tt)} you would cross it with a homozygous recessive (tt) in what is called a test cross. If all the plants are tall then the unknown parent is homozygous dominant (TT). If half the plants are short, then the unknown parent must be heterozygous (Tt). Homozygous Dominant Unknown Parent T T t Tt Tt t Tt Tt ALL TALL Heterozygous Unknown Parent T t t Tt tt t Tt tt 50% tall 50% short Incomplete Dominance In some organisms, however, an individual displays a trait that is intermediate between the two parents, a condition known as incomplete dominance. For example, when a snapdragon with red flowers is crossed with a snapdragon with white flowers, a snapdragon with pink flowers is produced. Neither the red nor the white allele is completely dominant over the other allele. Cholesterol levels in humans are a condition controlled by alleles that are incompletely dominant. Genotype Expressed Phenotype NN Normal cholesterol levels Nn Slightly elevated cholesterol levels nn Greatly elevated cholesterol levels If two parents with slightly elevated cholesterol levels (Nn) marry, their children could have three different levels of cholesterol. N N n NN Nn 25% would have normal cholesterol 50% would have slightly elevated levels N N Nn Tt nn Tt 25% would have greatly elevated levels Dihybrid Crosses You must be able to read a Punnett square showing a dihybrid cross like the one below. Purple (P) is dominant to Yellow (p) and Smooth texture (S) is dominant to shrunken (s) In a cross between parents that are heterozygous for both traits (PpSs) you would expect the following genotypes. 9 Purple and Smooth 3 Purple and Shrunken 3 Yellow and Smooth 1 Yellow and Shrunken Sex Linked Inheritance If a trait is autosomal, it will appear in both sexes equally. Recall that an autosome is a chromosome other than an X or Y sex chromosome. If a trait is sex-linked, it is usually seen only in males. A sex-linked trait is a trait whose allele is located on the X chromosome. Most sexlinked traits are recessive. Because males have only one X chromosome, a male who carries a recessive allele on the X or Y chromosome will exhibit the sex-linked condition. A female who carries a recessive allele on one X chromosome will not exhibit the condition if there is a dominant allele on her other X chromosome. She will express the recessive condition only if she inherits two recessive alleles. Thus, her chances of inheriting and exhibiting a sex-linked condition are significantly less. Dominant or Recessive? If the trait is autosomal dominant, every individual with the trait will have a parent with the trait. If the trait is recessive, an individual with the trait can have one, two, or neither parent exhibit the trait. Heterozygous or Homozygous? If individuals with autosomal traits are homozygous dominant or heterozygous, their phenotype will show the dominant characteristic. If individuals are homozygous recessive, their phenotype will show the recessive characteristic. Two people who are heterozygous carriers of a recessive mutation will not show the mutation, but they can produce children who are homozygous for the recessive allele. Hemophilia is a sex linked trait in humans that causes the blood to not clot properly What if a woman is a carrier for hemophilia XXh what would most likely be the patterns of inheritance of hemophilia in her children. h X X X XX X X Y XY X Y h h Because hemophilia is sex linked and recessive more of the more of the male children will be affected than female. Pedigree Analysis Imagine that you want to learn about an inherited trait present in your family. How would you find out the chances of passing the trait to your children? Geneticists often prepare a pedigree, a family history that shows how a trait is inherited over several generations. Pedigrees are particularly helpful if the trait is a genetic disorder and the family members want to know if they are carriers or if their children might get the disorder. Carriers are individuals who are heterozygous for an inherited disorder but do not show symptoms of the disorder. Carriers can pass the allele for the disorder to their offspring. I II III The gene for the ability to taste phenylthiocarbamide (PTC) is a dominant allele over the non-tasting gene. Notice that Female A in generation III has the ability to taste PTC, while Female B in the same generation does not. How many individuals in generation II can taste PTC? The trait in the above pedigree is autosomal and recessive because it appears in both sexes almost equally. If it were sex linked, it would appear more often in males than in females. DNA Fingerprinting Mrs. Brown had baked a birthday cake for her husband and then left it in a tin on the kitchen table. When she returned from shopping all that was left in the cake tin was a few crumbs and a smudge of blood where the thief had snagged their finger on the sharp edge of the tin. Below are the DNA fingerprints of her four children. Which one of the children’s DNA fingerprints most closely matches the blood stain and therefore is most likely to be the thief? The correct answer is John because he shares 9 alleles with the blood stain. • Bob only shares 7 alleles • Sue share 4, and • Lisa shares 4 Important Things To Remember Genes can be autosomal (all of our body cells) or sex cells (gametes / sperm or eggs). Only traits carried in the sex cells (gametes/sperm and egg) can be passed down from one generation to the next. A mutation in a skin cell, blood cell or muscle cell will not be passed down to the next generation. Genetic variations can help an organism survive when the environment changes. A mutations that leads to pesticide resistance might lead an insect to better survive than one that does not receive the mutated allele.