11.2 sex-linked

• Biologie
• Middelbare school
• vwo
• Leerjaar 4

Inheritance & Evolution

Homework

Biozones 159, 160, 161, 164

Assessment questions Glencoe page 319, questions 10-19

Learning objectives

You can distinguish between autosomal genes and sex-linked genes

You can explain how the X and Y chromosome determine sex

You can apply the laws of inheritance to sex-linked genes and crosses

The X-chromosome holds way more genes than the Y-chromosome. 

Some disease and syndromes linked to the X-chromosome: 

- colour blindness

- hemophilia

- Duchenne muscular dystrophy

- deficiencies

Important gene on the Y-chromosome

SRY-gene: gene that regulates many other genes

If the SRY-gene is present a mammal embryo will develop into a male

The gene coding for the proteins in the cones on your retina that help you distinguish between red and green is on the X-chromosome.

How many alleles of this gene will a woman have? Only write down a number.

The gene coding for the proteins in the cones on your retina that help you distinguish between red and green is on the X-chromosome.

How many alleles of this gene will a man have? Only write down a number.

There is a difference in the number of alleles between men and women for genes on the X-chromosome (sex-linked genes). Therefore we use a different notation. 

XX for women

XY for men

On the X-chromosome we note in superscript which allele is present on that chromosome. Still a capital letter for dominant and small letter for recessive. 

You still write down the Y-chromosome to show it is a male, but it will not have an allele in superscript.

Complete on paper and upload a picture:

A heterozygous, colourblind woman has children with a colourblind man. This form of colourblindness is sex-linked recessive. Draw the punnett square of this cross.

Use your punnett square from the previous question:

What is the chance of a colourblind child?

1. 0%
2. 25%
3. 50%
4. 100%

Use the same punnett square again

What is the chance of a daughter being colourblind?

1. 0%
2. 25%
3. 50%
4. 100%

Use the same punnett square again:

What is the chance of a son being colourblind?

1. 0%
2. 25%
3. 50%
4. 100%

Important points sex-linked inheritance: 

• Women have two X-chromosomes and because of that two alleles 
• Men have one X-chromosome and because of that one allele 
• It is possible that the chances of a particular trait are different for men and women 
• READ carefully what they are asking 
• DONT FORGET to also write down the X and Y chromosomes and not only the alleles 
• If the trait is recessive males are more likely to have it than females 
• If the trait is dominant females are more likely to have it than males 

Identifying sex-linked inheritance

Men only have 1 allele. Men ALWAYS receive their X-chromosome from their mom

The allele that fathers have they will always pass on to their daughters. 

In pedigrees with a sex-linked trait, the following rules apply:

- Dominant father = dominant daughters

- Recessive mother =  recessive sons

Upload a picture of your answer:
The grey symbols are people with a disease that is caused by a sex-linked gene. The red symbols are not sick. 7 and 8 are having a child, 11. 11 turns out to be a girl. What is the chance she has the disease?
And what if 11 was a boy?

In Drosophila (fruit flies) there is a recessive allele that causes veinless wings. This gene is located on the X-chromosome. A female with veinless wings is crossed with a male with normal wings. The F1 offspring that come from this cross and used to cross with each other to form the F2 generation. Which percentage of the F2 females will have veinless wings?

11.3 prenatal diagnostics 

see previous LessonUp or Table 5, page 315

-amniocentesis

- chorionic villus sampling

- fetal blood sampling

- NIPT test https://www.pns.nl/nipt

Karyogram

Chromosome portrait

Identify:

- Missing chromosomes

- Extra chromosomes

- Gender

Male

Female

Trisomy 21

Trisomy 13

Trisomy 18

Missing chromosome 11

Missing chromosome 19

Male

Female

Trisomy 21

Trisomy 13

Trisomy 18

Missing chromosome 11

Missing chromosome 19

Male

Female

Trisomy 21

Trisomy 13

Trisomy 18

Missing chromosome 11

Missing chromosome 19

https:

Homework

Biozones 164, 194, 195

Assessment questions Glencoe page 320: 21-33