Sex Chromosomes (4)

Question 1

What are autosomes?

Answer 1

Of the 23 pairs of chromosomes in a human cell, 22 pairs have more or less the same shape, i.e. are homologous. They are called autosomes.

Question 2

What are gonosomes?

Answer 2

The two chromosomes making up the 23rd pair differ in shape and are the sex chromosomes or gonosomes. They determine the sex of the offspring.

Question 3

The longer sex chromosome is called an __ chromosome.

Answer 3

X

Question 4

The shorter chromosome is called a __ chromosome.

Answer 4

Y

Question 5

Females have two X chromosomes in their cells, so they have the genotype ___________.

Answer 5

XX (homogametic)

Question 6

Males have one X chromosome and one Y chromosome in their cells, so they have the genotype _____.

Answer 6

XY (heterogametic)

Question 7

If the sex chromosomes are __ the undifferentiated sex organ (gonad tissue) in a very young embryo will develop into an ___ and the fetus will grow into a ____.

Answer 7

XX
Ovary
female

Question 8

If the sex chromosomes are ___, the presence of a gene on the Y chromosome will trigger the development of the ___ tissue into a testis and the fetus will grow into a ___

Answer 8

XY
Gonad
male

Question 9

What is the sex of an organism determined by?

Answer 9

The sex of an organism is therefore determined by the absence of the or presence of the Y chromosome.

Question 10

At what stage of development does this switch occur?

Answer 10

This ‘switch’ occurs when the embryo is 7 to 8 weeks old. After this many other genes, mainly on autosomal chromosomes, control the detailed differentiation of male and female characteristics of the fetus.

Question 11

How do mammals differ from birds and butterflies?

Answer 11

In mammals, males always have XY sex chromosomes, i.e. are heterogametic. In birds and butterflies, it is the females that are heterogametic.

Question 12

Inheritance of sex is a special form of monohybrid inheritance. How can you work out sex inheritance? You can work out sex inheritance in just the same way as for any other characteristic, but using the letters, X and Y as symbols to describe whole chromosomes instead of individual alleles.

Answer 12

You can work out sex inheritance in just the same way as for any other characteristic, but using the letters, X and Y as symbols to describe whole chromosomes instead of individual alleles.

Question 13

The Y chromosome is very short and has very few ___on it other than those responsible for ‘maleness’.

Answer 13

alleles

Question 14

The X chromosome is much longer and can carry many ____ along its length in addition to those for ‘femaleness’.

Answer 14

alleles

Question 15

True or false
Only a very small part of the X and Y chromosomes pair up during meiosis (i.e. have complementary genes) and crossing over occurs.

Answer 15

False,
Only a very small part of the X and Y chromosomes pair up during meiosis (i.e. have complementary genes) and NO crossing over occurs.

Question 16

What are sex-linked alleles?

Answer 16

The alleles that are carried on the non-homologous part of an X chromosome are called x-linked genes or sex-linked alleles.

Question 17

How many sex-linked alleles do men have?

Answer 17

Men have only one X chromosome so they will have only one of each sex-linked allele that is carried on the X chromosome.

Question 18

How many sex-linked alleles do women have?

Answer 18

Women with two X chromosomes will have two sex-linked alleles; one on each X chromosome.

Question 19

Alleles on the X chromosome therefore can be inherited by ____ ___, but the male can only receive one of the possible alleles.

Answer 19

either sex

Question 20

The female, with XX chromosomes, can be either homozygous or heterozygous for the ____ allele.

Answer 20

X-linked