Predicting
Inheritance
Genes are sections of DNA which code for a protein. When the gene is expressed it create a change in your physiology. Changes that we can see (eye colour, hair colour, number of fingers) are called Phenotypes.
Let go back a second to chromosomes. We know that you have half a set (23) from your mother, and another half a set (23) from your father.
Lets look at chromosome 1. In humans, this chromosome contains hundreds of genes. Lets say, for example, it contains the gene for hair colour.
Your mother has blonde hair, while your father has black hair. This means chromosome 1 from your mother has a blonde hair gene, while chromosome 1 from your father has a black hair gene.
This variation in a gene is called an allele.
An allele is a different version of the same gene.
You get one copy of every gene from parent A, and another copy of the same gene from parent B.
But it’s not that simply. Have you ever seen some one with parents who have Black hair and brown eyes, but they have Blonde hair and blue eyes.
This is because, some alleles are Dominant over others, meaning they overshadow the other allele. The allele not show is called Recessive.
You can only have 2 copies of a gene (one from mum and one from dad). This means, that if you have one dominant gene and one recessive gene, you’ll only show the dominant gene.
In order to show the recessive trait, you need to have 2 copies of it. This is why you may look more like your grandparents than your parents.
How can we determine, who gets what?
The best way for us to do this is to use a punnett square.
A punnett square is a great tool to predict the possible genotype and phenotype of the offspring.
In a simple punnett square (like the one to the right) we can predict 4 possibilities for one couple.
How do we create a punnett square?
Step 1 is to find out the genotype of the mother and father. The genotype means, what are their allele combinations.
Lets keep this simple and use hair colour as the example.
In the example to the left, we have two individuals who are Homozygous (meaning they have 2 of the same alleles).
Click next to see the combinations of gametes they can form.
They can form any one of those potential 4 combinations.
All the gametes, in this case, are Heterozygous (meaning they have 2 different allele combinations)
In the case above, the ‘B’ stood for the dominant black hair gene, while ‘b’ stood for the recessive blond hair gene. This means all their offspring will have Black hair.
If we mix 2 Heterozygous people together, we get a different combination of alleles. The punnett square to the right shows two heterozygous people. Although both people may have black hair, they carry the gene (allele) for blond hair.
This means 75% of their offspring will have black hair, and 25% will have blonde hair.
Look at your thumb. Give your self a thumbs up.
Does your thumb go straight up, or is there a bend in it?
The allele for the straight thumb is dominant (T), while the allele for a curved thumb is recessive (t).
Meaning if you have a curved thumb, you have two recessive alleles (tt)
Now, look at your fingers.
Hair or not?
The allele which causes hair to grow on the middle of your fingers is dominant (H). The allele for no hair is recessive (h).
If you have no hair your genotype for this allele is (hh).
Curly and straight hair is an interesting one. Unlike the above, curly and straight hair are called incomplete dominance. This means, unlike the other types, you can have a mix of the two.
AA means you have curly hair, Aa is a mix of the two, giving you wavy hair and aa gives you straight hair.
Questions
1. Draw a punnett square for the following crosses
a. If a brown eyed male (FF) has children with blue eyed female (ff) what is the % of a child with blue eyes.
b. If a red haired male (rr) has children with a black hair female (Rr) what is the % of the children have red hair
c. If a male with 6 fingers (bb) has children with a female with 6 fingers (bb) what is the % that 2 of their children will have 6 fingers.
2. Cystic fibrosis (CF) is caused by a recessive allele. You need to inherit two copies of the faulty allele to have CF. If you just have one copy of the faulty allele, you are a carrier but have no symptoms.
a. Explain what is meant by the term carrier?
b. Jeff wants to have children with Jess. However Jeff is diagnosed CF. They conduct a test on Jess and determine that she is a carrier for CF. What is the % that their child will have CF and won’t have CF.
3. Jack’s mother was recently diagnosed with Huntington’s disease (Jack’s mother is heterozygous). However his father does not have the disease, and has no family history of the disease.
Research Huntington’s disease and answer the questions bellow
a. What are 2 symptoms of Huntington's diseases?
b. Is Huntington’s a dominant or recessive disorder?
c. What is the % chance that Jack will have Huntington’s disease (draw a punnett square to illustrate this).