U2)KA3) variation and sexual reproduction- sex determination

Question 1

how is the sex of birds, mammals and insects determined

Answer 1

by the presence of sex chromosomes

Question 2

what are the sex chromosomes in animals

Answer 2

• organisms have 2 full sets of homologous chromosomes ,(diploid organisms) however one pair is an exception (the sex chromosomes)
• females have XX and males have XY

Question 3

during meiosis in the male, X and Y pair up because there is…..

Answer 3

• there is a small area of homologous genes near the centromere

Question 4

explain what is meant by heterogametic and homogametic

Answer 4

• females have two x chromosomes (XX)-(homogametic) they are called homogametic because all of their eggs contain an X chromosome.
• males have an x and a y (XY) heterogametic - they are called heterogametic because 50% of their sperm cells contain an X chromosome and the other 50% contain Y

Question 5

in most mammals what is on the the Y chromosome that causes the embryo to develop male characteristics

Answer 5

the Y chromosome carries the SRY gene . determining the development of male characteristics, it does this by encoding and expressing a protein called testes determining factor (TDF)
- if there is no SRY gene then the default pathway causes the embryo to develop as a female

Question 6

what can result in sex linked patterns of inheritance

Answer 6

-in males , X chromosome has many genes which do not have homologous alleles on the smaller Y chromosome, leading to sex linked patterns of inheritance.
(xy lacks most of corresponding homologous alleles on shorter Y chromosome)

Question 7

sex linked pattern of inheritance due to y being shorter example

Answer 7

• carrier females (XBXb) (the recessive b can be masked by dominant B) can pass on recessive deletrious alleles to give affected males (XbY) The recessive allele here cant be masked in males
• eg dominant allele R gives red eyes, recessive gives white in drosphilia
• the heterozygous genotype is XRXr, individuals with this genotype are described as carrier females
• a male with the recessive r will be shown as XrY , so will have white eyed phenotype because the smaller Y chromosome has no homologous gene to mask the effect.

Question 8

what is X chromosome inactivation

- when does it occur and what does it mean

Answer 8

• a process by which most of , most of the genes on the X chromosomes are inactivated randomly in each cell
• occurs during early embryonic development in homogametic females , means the cells have a single working copy of the X chromosome genes

Question 9

what does the X chromosome inactivation do that is beneficial to cells

Answer 9

• the X chromosome inactivation means females will not get a double dose of gene products , so ensuring that females have exactly the same level of gene products as a male , as a double dose may be harmful to cells

Question 10

why are females less likely to be affected by a deleterious allele

Answer 10

• females who inherit a deleterious allele (wheter dominant or recessive) on one of their X chromosomes , are less likely to be affected by its effects- because x chromosome inactivation is random about half the cells in any tissue will have a working copy of the sex linked gene
• the phenotype of males always shows the effects of the genes that are inherited on the X chromosome

Question 11

for some species ______ factors rather than genetic factors determine sex and sex ratio

Answer 11

environmental

Question 12

what is sex determination in reptiles controlled by

Answer 12

• environmental temperature of egg incubation

- incubation of eggs at less than 31 degrees produces all male offspring, and above 32 degrees all females

Question 13

sex can also change within individuals of some species as a result of _____ , ______ or _____ _____

Answer 13

• size, competition, parasitic infection

Question 14

sex change and determination in species due to environmental factors examples

• size
• competition
• parasitic infection

Answer 14

• size : clown fish : they live in groups with one large female and some smaller females. when the large female is removed , the largest male becomes a female
• competition: lesser mouse lemur : if a solitary female detects urine she produces more male offspring
• parasitic infection: insects infected by wolbachia bacteria : bacterial infection of the eggs kills the males or feminises them

Question 15

what is a hermaphrodite

Answer 15

hermaphrodites are species that have functioning male and female reproductive organs in each individual

Question 16

each produces male and female gametes and usually has a partner , with which to _____ gametes

Answer 16

exchange

Question 17

what is the benefit to individual hermaphrodites

Answer 17

• if the chance of encountering a partner is an uncommon event , there is no requirement of that partner to be of opposite sex