100% were red eyed = red is dominant - He then bred 2 offspring from the F1 generation F1 = Rr x Rr

Transcription

1 7. Gene Linkage and Cross-over Thomas Hunt Morgan 1910 Working with fruit flies he proved that genes on the same chromosome tended to be inherited together. = Linked genes ie. Eye color and hair color However, Morgan also noticed that he did not always get typical Mendelian ratios for his dihybrid crosses (9:3:3:1) He reasoned this was due to crossing over that occurs during meiosis - when paired, homologous chromosomes twist together (synapse), pieces of genetic material break off and reattach in new places - these new genes then are called recombinants - zygotes formed from these gametes will have new phenotypes The placement locus of the gene on a chromosome determines the chances of a cross over events for that gene - the longer the chromosomes, and the closer the gene is to the end of the chromosome, the greater the chance for a cross over Sex Linkage Morgan also noticed that sometimes only males would express a recessive trait and females wouldn t in the normal ratio so he did test crosses of fruit flies for eye color - He crossed a red eyed female with a white eyed male P1 = RR x rr 100% were red eyed = red is dominant - He then bred 2 offspring from the F1 generation F1 = Rr x Rr Ratio of Red eyes to White eyes = 3:1 but all the white eyed flies were MALE

2 Morgan concluded: - of the 4 sets of fruit fly chromosomes, only 3 are identical ( autosomal) - the fourth set were different in males and in females = sex chromosomes Males: XY Females: XX - Y chromosomes are smaller and carry very few genes (except sex determination), - Males can contribute an X or a Y to offspring but females can only contribute an X = males determine the sex of the offspring - Traits that are found on the X chromosome are called Sex Linked Traits = X-Linked Traits - Humans = 46 chromosomes: 22 pairs of autosomes, 1 set of sex chromosomes In this class, we assume the Y chromosome carries no genes. For crosses, determining probability of sex linked traits is written as follows: Ie. In humans, color blindness is a sex linked trait, males who carry the gene will be color blind because they only have one X chromosome B = normal sight b = color blind A female who is heterozygous for color blindness marries a color blind man. What are the phenotypic and genotypic ratios of their offspring. P1 = X B X b x X b Y Phenotypes: normal female colorblind female normal male colorblind male Ratio: Genotypes: heterozygous homozygous cb homozygous normal homozygous cb Ratio: What are the chances of having a colorblind child, either sex? What are the chances of having a colorblind boy? **Note: Phenotypic Ratios are affected because of the different sexes.

3 7. Gene Linkage and Cross-over Thomas Hunt Morgan 1910 Working with fruit flies he proved that genes on the same chromosome tended to be inherited together. = ie. Eye color and hair color However, Morgan also noticed that he did not always get typical Mendelian ratios for his dihybrid crosses (9:3:3:1) He reasoned this was due to that occurs during meiosis - when paired, homologous chromosomes twist together, pieces of genetic material break off and reattach in new places - these new genes then are called - zygotes formed from these gametes will have new The placement locus of the gene on a chromosome determines the chances of a cross over events for that gene - the longer the chromosomes, and the closer the gene is to the end of the chromosome, the greater the chance for a cross over Sex Linkage Morgan also noticed that sometimes only males would express a recessive trait and females wouldn t in the normal ratio so he did test crosses of fruit flies for eye color - He crossed a red eyed female with a white eyed male P1 = RR x rr 100% were red eyed = red is dominant - He then bred 2 offspring from the F1 generation F1 = Rr x Rr Ratio of Red eyes to White eyes = 3:1 but all the white eyed flies were MALE

4 Morgan concluded: - of the 4 sets of fruit fly chromosomes, only 3 are identical - the fourth set were different in males and in females = Males: XY Females: XX - Y chromosomes are smaller and carry very few genes (except sex determination), - Males can contribute an or a to offspring but females can only contribute an = determine the sex of the offspring - Traits that are found on the X chromosome are called = - Humans = 46 chromosomes: 22 pairs of autosomes, 1 set of sex chromosomes In this class, we assume the Y chromosome carries For crosses, determining probability of sex linked traits is written as follows: Ie. In humans, color blindness is a sex linked trait, males who carry the gene will be color blind because they only have one X chromosome B = normal sight b = color blind A female who is heterozygous for color blindness marries a color blind man. What are the phenotypic and genotypic ratios of their offspring. P1 = X B X b x X b Y Phenotypes: normal female colorblind female normal male colorblind male Ratio: Genotypes: heterozygous homozygous cb homozygous normal homozygous cb Ratio: What are the chances of having a colorblind child, either sex? What are the chances of having a colorblind boy? **NOTE:

5 Gene Linkage and Sex Linked Traits Worksheet 1. What are the sexes and eye colors of flies with the following genotypes? X R X r X R Y X r X r X R X R X r Y 2. What are the genotypes of these flies: white eyed, male white eyed, female red eyed female (heterozygous) red eyed, male 3. Show a cross between a pure red eyed female and a white eyed male. What are the genotypes of the parents: and How many are: white eyed, male white eyed, female red eyed, male red eyed, female 5. Show the cross of a red eyed female (heterozygous) and a red eyed male. What are the genotypes of the parents? & How many are: white eyed, male white eyed, female red eyed, male red eyed, female 4. Show the cross of a white eyed female X r X r with a red-eyed male X R Y. Math: What if in the above cross, 100 males were produced and 200 females. How many total red-eyed flies would there be?

6 5. A recessive sex-linked gene (h) located on the X chromosome increases blood clotting time. This causes the genetic disease, hemophilia. Complete the table below Male or Female? Has Trait? (Y or N) Carrier? (Y or N) X H X H X H X h X h X h X H Y X h Y 6. A heterozygous woman marries a non-hemophilic man. What are their chances of having: a) Hemophilic girls b) Hemophilic boys 7. Baldness is a sex-linked trait. A heterozygous female maries a normal male. What is the chance of them having a bald child, either male or female? 8. Why are there so few bald female born? Explain using genetics. 9. In cats, coat color is a sex linked trait. A black coat is due to a pair of genes, BB, while bb produces a yellow coat. The coat color calico is Bb. A yellow female is bred to a black male. What colors of cats will be born?