Learning Objective

In this lesson we will learn how traits controlled by genes located on sex chromosomes follow different patterns of inheritance in males and females.

Learning Outcomes

  • Explain what a sex-linked trait is and differentiate between X-linked and Y-linked traits.
  • Write genotypes for sex-linked traits and describe hemizygosity in males.
  • Determine genotypes and phenotypes for X-linked and Y-linked traits in males and females.
  • Compare sex-linked inheritance in males and females.

 

 sex-linked inheritance worksheet  year 10 biology pdf workbook

Click images to preview the worksheet for this lesson and the Year 10 Biology Workbook.

 

Introduction

  • Unlike autosomal chromosomes, X and Y sex chromosomes are not homologous, which means they are different sizes and contain different sets of genes.
  • Traits controlled by genes located on sex chromosomes are called sex-linked traits.
  • Traits controlled by genes on the X chromosome are called X-linked traits.
  • Traits controlled by genes on the Y chromosome are called Y-linked traits.
  • Males (XY) and females (XX) have different numbers of alleles for genes located on sex chromosomes. Therefore, inheritance of sex-linked traits (sex-linked inheritance) follows different patterns in males and females.
  • Since females don’t possess Y chromosomes and the X chromosome is much larger than the Y chromosome, most cases of sex-linked inheritance are X-linked.

 
x and y chromosomes size comparison

X and Y chromosomes are different sizes and contain different sets of genes.

(Image: nathan, Adobe Stock)

 

X-Linked Inheritance in Males and Females

  • For X-linked genes:
  • Males have one allele as they have one X chromosome.
  • Females have two alleles as they have two X chromosomes.
  • Since males only have one allele, it is not possible for males to be homozygous or heterozygous for X-linked genes. Instead, they are always what is referred to as hemizygous.
  • For X-linked traits, males simply express the phenotype of whichever allele they are hemizygous for.
  • Since there are no male heterozygotes, there is no distinction between dominance, incomplete dominance and codominance for X-linked traits in males.
  • Since females have two alleles for X-linked genes, X-linked traits are expressed in the same way as autosomal traits in females.
  • Males inherit their X chromosomes from their mothers. Therefore, all X-linked alleles in males are maternally inherited.

 
x-linked genes in males and females

Males can only be hemizygous for X-linked genes.
Females can be homozygous or heterozygous for X-linked genes.

 

Y-Linked Inheritance in Males and Females

  • For Y-linked genes:
  • Males have one allele as they have one Y chromosome.
  • Females have no alleles as they have no Y chromosomes.
  • Since males have one allele for Y-linked genes, Y-linked traits are expressed in the same way as X-linked traits in males.
  • Since females do not have any alleles for Y-linked traits, Y-linked traits do not exist in females.
  • Males inherit their Y chromosomes from their fathers. Therefore, all Y-linked alleles in males are paternally inherited.

 
y-linked genes in males and females

Males can only be hemizygous for Y-linked genes.
Females do not possess Y-linked genes.

 

Writing Genotypes for Sex-Linked Traits

  • To distinguish between sex-linked alleles and autosomal alleles, sex-linked alleles are written as superscript letters in combination with an X or Y.
  • For X-linked dominant traits:
  • Males have the following possible genotypes:
  • XAY (hemizygous dominant)
  • XaY (hemizygous recessive)
  • Females have the following possible genotypes:
  • XAXA (homozygous dominant)
  • XAXa (heterozygous)
  • XaXa (homozygous recessive)

 
genotypes for x-linked traits in males and females

 

  • For Y-linked traits:
  • Males have the following possible genotypes:
  • XYA
  • XYa
  • Females have only one possible genotype:
  • XX

 
genotypes for y-linked traits in males and females

 

Summary

  • Sex-linked traits are controlled by genes located on sex chromosomes.
  • X-linked traits are controlled by genes on the X chromosome.
  • Y-linked traits are controlled by genes on the Y chromosome.
  • Sex-linked inheritance follows different patterns in males and females as they have different numbers of alleles for sex-linked traits.
  • Most cases of sex-linked inheritance are X-linked.
  • For X-linked genes, males have one allele (from their mother) and females have two alleles.
  • Males are hemizygous – they cannot be homozygous or heterozygous. They express the phenotype of whichever allele they possess.
  • Females express X-linked traits in the same way as autosomal traits.
  • For Y-linked genes, males have one allele (from their father) and females have no alleles.
  • Males express Y-linked traits in the same way as X-linked traits.
  • Females do not possess Y-linked traits.
  • Sex-linked alleles are written as superscript letters in combination with an X or Y.

 
ishihara colour blindness test

Do you see a number in this picture? Colour blindness is linked to a gene on the X chromosome. It causes individuals to have difficulty differentiating between certain colours, such as red and green.

(Image: Desconocido, Wikimedia Commons)

(Header image: Atypeek Design, Adobe Stock)

 

 sex-linked inheritance worksheet  year 10 biology pdf workbook

Click images to preview the worksheet for this lesson and the Year 10 Biology Workbook.