lecture 6- x and y linked traits, non-disjunction, and sex determination

Question 1

x-linked traits in humans

Answer 1

• the homogametic sex (XX) can be a carrier of recessive trait
• the heterogametic sex (XY) will express recessive trait if it carriers the allele responsible because it is hemizygous
• females are XX and males are XY, so males receive their only X chromosomes from thier mom

Question 2

sex-linked traits in humans

Answer 2

• genes or sex chromosomes show non-mendelian pattern or inheritance
• pedigree analysis should consider possible X-linked inheritance
• it is sometimes difficult to find a pedigree with enough affected individuals to exclude other models
• the same trait can be governed by multiple genes

Question 3

x-linked recessive inheritance

Answer 3

• trait more common in males than in females
• for rare phenotypes, children of affected males are not affected, but 1/2 of male children of daughters for affected males may show the trait- it skips a generation
• it is more common in male than in females because they only have 1 x chromosome

Question 4

x- linked dominant inheritance

Answer 4

• males pass the trait to all their daughter but none of their sons
• affected females married to normal males pass their trait to 1/2 of their sons and daughters

Question 5

y-linked traits in humans

Answer 5

• y-linked traits are “holandric” which means it is entirely male
• TDF causes sex determination
• such traits should be transmitted to all sons

Question 6

sex determination

Answer 6

• organisms have evolved different forms of sex determination
• sex determination could be determined by genes
• genotypes determin sex
• genes present in differential region

Question 7

sex factors

Answer 7

• maleness determined by testis determining factor (TDF) encoded by Sry gene on the Y chromosome
• Sry is a transcription factor expressed just before testis formation
• TDF directs gonad differentiation to testes rather than ovaries; other sex differences are due to hormones or other gonad- produced factors

Question 8

determination of maleness in humans

Answer 8

• the TDF region was identified by studying XX individuals who developed as males and XY individuals who developed as females
• XX males had Y chromosome fragment transferred to one of their X chromosomes and XY females lacked this fragment on their Y chromosome

Question 9

non-disjunction

Answer 9

• happens when paired homologs (meiosis 1) or sister chromatids (meiosis ii) erroneously move to the same pole- they fail to disjoin

Question 10

sex determination factors

Answer 10

• environment determines sex - environment conditions sex

Question 11

sex determination in drosophila

Answer 11

• xxy is female, xo is male
• hermaphrodites (with male and female organs)