Genetic Problems Worksheet

Genetic problems Worksheet (2 points each) 1. Sometimes crossing two heterozygotes can lead to something called “homozygous lethal” that causes the offspring homozygous for one of the alleles to die before birth. In a rare skin disease, a particular skin development gene (Ss) exhibits alterations. A mouse homozygous for the s allele will have normal skin, heterozygotes have tough skin, and all homozygous S die before birth. Predict the phenotypic ratio from crossing two Ss mice? The phenotypic ratio for crossing two Ss mice would be 1:2:1, meaning that there will be one homozygous SS mouse, two heterozygous Ss mice, and one homozygous ss mouse. S S SS s Ss s Ss ss 2. For the following two questions we’re talking about a flowering plant, Fakeus madeupticus: red (r) is the recessive phenotype and white (R) is the dominant phenotype. They simply work as basic dominant and recessive alleles. a) Plant 1 (♀) is red and plant 2 (♂) is white, this is the P1 generation. When we crossed them, we got all white plants, the F1 generation. What is the genotype of each parent plant? Plant 1 genotype: rr (red) Plant 2 genotype: RR (white) r R Rr R Rr r Rr Rr b) The F1 generation was then crossed and there were 100 plants in this F2 generation. About how many white plants do you expect? How many red ones? Red=1/4=25 White=3/4=75 R R RR r Rr r Rr rr c) Based upon your basic knowledge of Mendelian genetics and chi-square tests (considering your answers from 2a and 2b), would the following distribution for the F2 generation be statistically similar or different after a chi-square test: 35 red and 34 white. Why? It is insignificant because during the cross between the F1 generation, ¾ of the offspring are red and ¼ are white. Yet the results make it seem the ratio is 1:1. d) If the distribution was 46 red and 49 white, what are the likely genotypes of the two parent plants? a. Plant 1: rr (red) b. Plant 2: Rr (white) 3. What are two reasons why elephants would be a bad choice for studying genetic inheritance? Elephants have a large reproductive cycle, meaning that noting observational changes takes too much time. Elephants also only produce one offspring per cycle; therefore, the size of offspring is too low to analyze. 4. A TT (tall) plant is crossed with a tt (short) plant: a. Show a punnett square of this cross. T T t Tt Tt t Tt Tt b. What percentage of the offspring will be tall 100% c. What percentage of the offspring will be short 0% d. What percentage of the offspring will be heterozygous 100% e. Homozygous 0% 5. Briefly, explain why it is advantageous to have 2 alleles per trait. It is advantageous to have two alleles per trait because in the instance of a deleterious mutation, an organism that is a diploid has a higher chance of having a working allele and being able to survive. 6. Down’s syndrome affects one in 700 children and is caused by having 3 copies of chromosome 21 (for a total of 47 chromosomes) in every cell. Explain how this could occur. (Hint: specify during which parts of meiosis the error may have occurred. You may find it useful to draw diagrams.) Down’s syndrome is defined as a chromosomal disorder where there is an extra copy of chromosomal number 21 because of atypical cell division occurring. While mitosis and meiosis are happening, a pair of chromosomes do not separate evenly, and this causes an extra section of chromosome to form. This is called non-disjunction. 7. In Guinea pigs, the allele for short hair (S) is dominant. a. What genotype would a heterozygous short haired guinea pig have? Hh b. What genotype would a long-haired guinea pig have? HH 8. Two short haired guinea pigs are crossed and 25% of the offspring have long hair. What are the genotypes of the parents? Show a Punnett square. Hh x Hh H H HH h Hh h Hh hh 9. In humans, sex genotypes are: XX=female, XY= male. What is the probability that any couple will have a female baby? Male baby? The secondary sex ratio is defined as 1.05, meaning that for every 100 girls born, there are 105 boys born, and the percentage of likelihood that a single baby will be a boy is 51.2%. 10. Given the above human gender genotypes, hemophilia is a X-linked (carried on the X chromosome) recessive genetic disorder that results in inefficient blood clotting. a. Explain why males are more likely to develop this genetic disorder than women? Males are more likely to develop this genetic disorder since they only have one X, compared to females who have two. Since there are two X chromosomes present in females, if the first one is mutated, there is another one to make up for the nutated X. This is not the case for males. b. Can males be carriers of this disease? meaning they carry the chromosome for the disorder but do not have the disease? Why or why not? Males cannot be carriers for this unless they are showing symptoms, because there is only one X present to carry the disease.