# 5.1.2007 - 5.1.2007 • If 1 is crossed with two what is...

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5.1.2007 If 1 is crossed with two what is the probability that the offspring expresses that trait (is homozygous recessive) If two parents can have an affected offspring, they must be heterozygous So what is the probability that the male is a heterozygote and that a female is a heterozygote You have to start with the known genotypes and work to those that you are trying to figure out Assume that people who are not affected and are not in the affected family that they married into the family and are homozygous for the most common trait** Dd Dd 3 DD Dd Dd D DD dd 1/3 DD, 2/3 Dd Dd DD DD Dd Dd Dd 1 ½ DD 2 ½ Dd Dd Dd Dd dd dd 1x2 dd o ½ = p (1 = Dd) o 1 = p (2=Dd) o ¼ = p (Dd x Dd p(dd) o Answer: 1/8 Now cross 1 and 3 to see probability that their offspring is affected o Only way they can have an affected offspring is if both are heterozygots o Probability of 1 being a heterozygote is still ½ as before o P(3=Dd) is ? o Probability of two heterozygotes getting dd ¼ o 3 cannot be homozygous recessive, that p = 0 Therefore the probability that she is heterozygous is 2/3 and homozygous dominant is 1/3 o So p(3= Dd) 2/3 o ½ x 2/3 x ¼ = 1/12 = answer Exceptions of Mendelian Genetics Learn the genetic diseases and genetic traits to the point that it just helps you remember what each phenomena is about and how it works, don’t just memorize them as pairs These are other things that are going on, additional things, not unusual or exceptions to rules (more often the rule) they are just in addition to Mendelian genetics, don’t refute them Lethal allele causes the death of an organism and cannot pass it on, almost always recessive o Exception: Huntington’s It is a dominant allele causing Huntington’s which is lethal Small number of heterozygotes in the population and very small amount of people who are homozygous dominant. Most people have homozygous recessive Onset is adult and post reproductive so individuals who have it don’t know they have it at the time they have kids so they pass on the allele and then start coming down with symptoms HH and Hh both die of Huntington hh lives o Manx tails in cats

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Cat having no tail If you cross two manx cats that are heterozygous Mm x Mm Normal allele has a gene product needed for normal development of the gene product Individuals with only half of this product (half the normal quantity of this product) develop half of a spinal column, which is enough for the neck and back but not the tail. No spine for the tail, no tail. Heterozygotes have half the normal gene product If you are homozygous for it you would die in gestation because you have no spine, and you only give progeny ratio for live births When considering a serious dominant genetic disease, assume that it is lethal in utero when homozygous Meaning lethal before birth or during gestation
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## This note was uploaded on 04/07/2008 for the course BIO 325 taught by Professor Saxena during the Spring '08 term at University of Texas at Austin.

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5.1.2007 - 5.1.2007 • If 1 is crossed with two what is...

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