Patterns of Inheritance

What are genes?

A gene is a section of DNA which is passed down from the parents to the offspring.

Each gene is used to make a unique trait. Different versions of the gene are known as alleles.

We receive one allele for each gene from our mother, and one allele of the gene from the father.

This what creates different phenotypes (physical appearances) and genotypes (genetic expressions) in the children to the parent.

What is the incomplete dominance pattern? 

Often, due to incomplete dominance, and neither allele is expressed physically but instead the offspring are a blend or an intermediate of its two parents. Both alleles have blended together. For instance, a brown- eyed mother and blue - eyed father may have children with green eyes. A tall father and a short mother has children between their heights.

What is the dominant v recessive pattern?

Sometimes, one allele is dominant and be expressed physically whereas the other one is recessive and does not show physically.  Examples of dominant traits include brown eyes, curly hair and freckles. In these cases, if one of the parents has the dominant trait, all the children will too.  Recessive traits only show physically in the offspring is both parents have the recessive trait.

What is the codominance pattern?

In codominance, both alleles equally dominate and are expressed at the same time. For example, a red and white flower offspring can be created from a red flower crossed with another white flower. Another example is the human blood type. Some people are AB. This means they express both A and B antigens on their red blood cells.

What is the X - linked pattern?

Humans have 46 chromosomes in the nucleus of every cell. These chromosomes are paired. The first 22 pairs are autosomal and determine body characteristics. The last or 23rd pair is sex linked and determines whether the human will be male or female. 

For the 23rd pair, males have an X and a Y chromosome. For females, they have two X chromosomes. The X chromosome is larger than the Y chromosome. Therefore, it contains more genes, especially for metabolism. 

In every female cell, only one X chromosome is activated randomly. The other X chromosome is inactivated. If one X chromosome contains a mutant gene, only some of the cells in the female body will express this mutant gene. 

In a male cell, the X chromosome is always expressed along with the Y chromosome. Therefore, if the X contains a mutant gene, all cells will be mutant. 

We call traits which are expressed by the X chromosome X - linked traits. Examples include haemophilia and colour blindness.

Your turn

Choose an X linked condition to do some research about. 

• Colour blindness 

• Haemophilia 

Questions: 

1. Is it an X - linked recessive or dominant condition? 

Colour blindness is an X linked recessive condition

2. If it is recessive, do carriers or heterozygotes show the condition? 

No they won’t show it. The carrier female has a 50% chance of passing the condition onto her son. 

3. How common is it in Australia? Is it more common in men or women?

About 550,000, 2.2%, 0.4% for women, 8% for men. 

4. Why is the condition concerning or dangerous? 

Colour blindness can make it difficult to do daily activities. This is because you can’t see colour and doing things like checking if food is safe or not is difficult, especially if meats are still raw or not.

5. Complete a punnett square for a healthy dad and a mother who is a carrier of the condition. 

c=colourblind gene (recessive)

	X^C	Y
X^C	X^CX^C	XCY
X^c	X^cX^C	XcY

6. Answer the following regarding the punnett square above: 

What is the chance of having a girl? 50%

What is the chance of having a girl who is colour blind? 0%

What is the chance of having a boy who is colour blind? 25%