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Describe human traits with only 2 alleles Describe traits with multiple alleles Explain blood-type alleles and heredity Describe sex determination Explain sex-linked heredity Define carrier and explain what it means to be a carrier Describe examples of traits with 2 alleles •Some human traits are controlled by a single gene with two alleles •Dimples •Cleft in the chin •Attached/unattached ear lobes •Widow’s peak Check: What is one human trait controlled by one gene with only two alleles? • Some human traits are controlled by single genes with multiple alleles • Example: Blood type (A, B, AB, O) • Most human traits are controlled by multiple genes that act together Explain blood-type alleles and heredity There are four types of blood – A, B, AB, and O Three alleles control human blood type: Two of the alleles are codominant: IA and IB One allele is recessive: i Explain blood-type alleles and heredity Possible genotypes: IA IB = AB blood type IA i or IAIA= A blood type IB i or IBIB = B blood type i i = O blood type CHECK: What are the three alleles that control human blood type? What are the four blood types? Which blood type alleles are codominant? If the genotype is ii what is the blood type? POINT > Explain blood-type alleles and heredity Suppose a heterozygote with type A blood has offspring with a heterozygote with type B blood. What type blood will the children have? A I i x B I i Type B heterozygous parent B I Type A heterozygous parent A I i A B I I A I B I ii i 25% type AB 25% type A 25% type B i 25% type O i CHECK: Suppose a homozygous woman with type A blood has offspring with a man with type AB blood. What are the genotype percentages of the offspring? A A I I x A B I I POINT > Describe sex determination Human body cells 23 pairs of chromosomes (= 46 total) 22 pairs of “regular” chromosomes are called autosomes 1 pair of sex chromosomes = X and Y Girls have two X chromosomes (XX) Boys have one X and one Y chromosome (XY) POINT > Describe sex determination During meiosis, the sex chromosomes split apart (like the other 22 pairs) In women, all egg cells have an X chromosome In men, half of the sperm cells get an X and the other half get a Y The male sperm cell (X or Y) determines the sex of the offspring The formation of egg and sperm cells: 1 sperm cell 4 sperm cells 1 egg cell 1 really good egg cell CHECK: How many autosomes do humans have? What are the sex chromosomes in a woman? What are the sex chromosomes in a man? Explain sex-linked heredity The genes for some human traits are carried on the sex chromosomes They are called sex-linked genes, and the traits are sex-linked traits Like other traits, they can have dominant and recessive alleles POINT > Explain sex-linked heredity FEMALES (XX) A dominant allele on one X will cover up a recessive allele on the other X XRXr If “r” is a recessive allele that causes a disease, this girl is not affected A girl would have to be XrXr to have the disease POINT > Explain sex-linked heredity MALES (XY) If there is a recessive allele on the X it will be expressed XrY This boy will have the disease Many recessive sex-linked traits carried on the X chromosome will show up more in males – ex. colorblindness, hemophilia CHECK: If “g” is a sex-linked recessive allele that causes a disease, which people would have the disease? G a)X Y g XY b) c) XGXg d) XGXG e) XgXg POINT > Define carrier and explain what it means to be a carrier A carrier is a person who has one recessive allele for a trait and one dominant allele (heterozygous) We usually use the term “carrier” to refer to a trait that is a recessive disease A carrier does not have the trait, but can pass on the recessive allele to offspring In the case of sex-linked traits, only females can be carriers POINT > Define carrier and explain what it means to be a carrier What if a female carrier for colorblindness has offspring with a colorblind man? What will the children have? C = allele for good color vision c = allele for colorblind C c X X x c X Y Female carrier C X Colorblind man c X c X C c X X c X c X Y C X c X Y Y 25% colorblind girls 25% carrier girls 25% good vision boys 25% colorblind boys CHECK: What is a carrier? Does a carrier have the disease? Homework Read 5.1 pages144-150 S.A. #1-3 GRAS pp. 69-71