sex linkage

Question 1

how many homologous pairs of chromosomes do humans have

Answer 1

22 homologous pair (autosomes)

Question 2

what is are the sex chromosomes for male humans

Answer 2

XY

Question 3

what are the sex chromosomes for female humans

Answer 3

XX

Question 4

how is sex genetically determined in mammals

Answer 4

sex chromosomes are X and Y
females - XX
all gametes have an X
gametes only have 1 chromosome as they are haploid
males - XY
50% sperm cell gametes have X chromosome, 50% sperm cell gametes have Y chromosome

Question 5

define sex linkage

Answer 5

any gene that is carried on the X or Y chromosome

Question 6

about 7% of males are affected by red green colour-blindness whereas only 0.4% females are affected
explain why

Answer 6

males only need one recessive b allele to be colour-blind
probability of both parents passing on recessive allele is a lot lower females need 2 recessive alleles due to having 2 X chromosomes
7% affected - doesn’t affect quality of life so it can be passed on

Question 7

define “selective removal” of gene from population

Answer 7

people die before having children so it can’t be passed on (lethal gene)

Question 8

why are males affected more often than females by sex linked disorders

Answer 8

X chromosome is longer than Y so carries more genes

for most of the X chromosome there is not a corresponding homologous part of Y chromosome

recessive alleles on this non homologous part of X chromosome will occur more frequently in males as there is no homologous part of Y chromosome to carry the dominant allele