1. Sex-linked and Polygenic Traits

2. Brainstorm and write down as many things as you can remember about chromosomes
What are chromosomes?
DNA that has been tightly wound up and condensed into a package
How many chromosomes do humans have?
23 pairs for a total of 46 chromosomes
Chromosomes are DNA that has been tightly wound up and packaged. Long DNA strands have been wrapped around proteins called histones and then coiled in on itself to form a tight, condensed structure.
In humans, there are typically 23 chromosome pairs, one set inherited from the mother and the other inherited from the father. So there is a total of 46 chromosomes.
What are Chromosomes?

3. One of these things is not like the other one…..
One chromosome is inherited from the father and the other is inherited from the mother
Chromosome pairs 1 to 22 are identical in shape
These are called autosomes
The autosomes are numbered based on their size. They are ordered from longest to shortest.
The first twenty-two chromosomes are identical in shape from the paternal and maternal origin (but of course the alleles vary).
The 23rd chromosome is different as this example has an X and a Y chromosome.
One of these things is not like the other one…..

4. One of these things is not like the other one…..
The 23rd pair of chromosomes is different for males and females
These are called the sex chromosomes
Females have two copies of the X chromosome
Males have an X and a Y chromosome
What is the gender of the organism this cell came from?
The first twenty-two chromosomes are identical in shape from the paternal and maternal origin (but of course the alleles vary).
The 23rd chromosome is different as this example has an X and a Y chromosome.
One of these things is not like the other one…..

5. Autosomes – pairs of chromosomes (one of paternal and one of maternal origin) with the same shape but varying alleles
Sex Chromosomes - either X or Y chromosome that determines the sex of an individual, with XX resulting in a female and XY resulting in a male
Karyotype – number and appearance of chromosomes in the nucleus of a eukaryotic cell (see right)
Definitions

6. The female can only donate an X chromosome.
Sex-linked trait – gene that is carried only by the male or female parent
This means that the trait is carried on either the X or Y chromosome
Therefore the trait is linked to the gender of the individual
If a gene is found only on the X chromosome and not the Y chromosome, it is said to be a sex-linked trait. Because the gene controlling the trait is located on the sex chromosome, sex linkage is linked to the gender of the individual.
Usually such genes are found on the X chromosome. The Y chromosome is thus missing such genes (See Diagram above.).
The result is that females will have two copies of the sex-linked gene while males will only have one copy of this gene. If the gene is recessive, then males only need one such recessive gene to have a sex-linked trait rather than the customary two recessive genes for traits that are not sex-linked. This is why males exhibit some traits more frequently than females.
The female can only donate an X chromosome.
If the father donates an X chromosome, the offspring will be female. If he donates a Y chromosome, the offspring will be male.
Sex-linked traits

7. Colour Blindness
What numbers do you see in the circles?

8. Statistics of Red-Green Colour Blindness
Protan – less sensitive to red light
Deutan – less sensitive to green light
Why are men affected at a much higher rate compared to women?
Roughly 8% of men and 0.5% of women are affected with colour blindness
Statistics of Red-Green Colour Blindness

9. What does this mean for whether or not the trait will be expressed?
Colour blindness is a recessive, sex-linked trait located on the X chromosome
Therefore, females will have two copies of the sex-linked genes
Males will only have one copy of the sex-linked genes
What does this mean for whether or not the trait will be expressed?
In order for the recessive trait to be expressed in females, they need to inherit two recessive alleles for colour blindness.
But, males only need one recessive allele of the sex-linked chromosome.
This is why males exhibit some traits more frequently than females
For example, the allele for colour blindness is located at A. This allele is only present on the X chromosome and not the Y. Therefore, a male with X and Y will only have the one allele to determine the ability to see colour. If he inherits one recessive allele, he will be colour blind for red and green.

10. Punnett Squares of Sex-linked traits

11. What is the resulting phenotypes of the males and females?
What is the result in offspring when the mother is colour blind and the father is not?
Colour blindness is a recessive, sex-linked trait located on the X chromosome
X b
X B Y
X B X b
X b Y
What is the resulting phenotypes of the males and females?
When the mother is color blind and the father is not:
all sons are going to be color blind
daughters are normal but carriers of the gene
Practice 1

12. What is the resulting phenotypes of the males and females?
What is the result in offspring when the father is colour blind and the mother is homozygous dominant?
Colour blindness is a recessive, sex-linked trait located on the X chromosome
X B
X b Y
X B X b
X B Y
What is the resulting phenotypes of the males and females?
When the father is color blind and the mother is not:
all sons are normal
all daughters are normal but carriers
Practice 2

13. Practice Worksheet
In pairs, complete the practice worksheet of sex-linked traits.

14. Are people either short or tall?
Unlike the peas used in Mendel’s experiments, people are not specifically ‘short’ nor ‘tall’
Height is measured on a range of values
The majority of the population have a height in-between
Are people either short or tall?

15. Polygenic Inheritance
Another exception to Mendel’s rules of inheritance
Occurs when a trait is controlled by two or more genes
Each dominant allele “adds” to the expression of the next dominant allele
Another exception to Mendel’s rules is polygenic inheritance, which occurs when a trait is controlled by more than one gene. This means that each dominant allele "adds" to the expression of the next dominant allele.
Polygenic Inheritance
Examples – height, skin colour, eye colour, weight

16. Usually traits are polygenic if there is a wide variation in the trait
Height is a trait that is controlled by at least 3 genes with 6 alleles
Offspring that are dominant for ALL the alleles for height will be very tall
Skin colour is another example of polygenic inheritance
Usually, traits are polygenic when there is wide variation in the trait. For example, humans can be many different sizes. Height is a polygenic trait, controlled by at least three genes with six alleles. If you are dominant for all of the alleles for height, then you will be very tall. There is also a wide range of skin color across people. Skin color is also a polygenic trait, as are hair and eye color.

17. Are there other factors we need to consider?
Suppose height is determined by genes A, B, and C. Offspring that are dominant for ALL the alleles will be very tall.
Which combination of alleles will produce the tallest offspring?
Which combination of alleles will produce the shortest offspring?
What height will the rest of the offspring demonstrate?
Each dominant allele adds to the expression of another dominant allele. Therefore offspring that are dominant for all the alleles will be very tall. So the offspring that is most likely going to be the tallest is the one with the genotype of AABBCC.
And therefore, the offspring that is most likely to be shorter is the genotype of aabbcc.
The rest of the offspring will be somewhere in the middle range.
It is important to note that polygenic traits are also impacted by the organism’s environment. For example, nutrition and living circumstances will affect how tall we will grow (even if our genotype is all dominant alleles).
As well, weight is affected by our nutrition and living environment. Skin colour is affected by sun exposure and other factors, etc.
Are there other factors we need to consider?

18. Represents Marriage
What is a pedigree?
A pedigree is a diagram showing the genetic relationships between members of a family
Used to analyze the patterns of inheritance for specific genetic traits
pedigree is a diagram showing genetic relationships between members of a family. It is used to analyze patterns of inheritance for specific genetic traits.
This information is background knowledge for the Pedigree Polygenic Trait activity that will be completed next.
Represents
Children
Males = rectangles
Females = circles

19. This is a more complex example demonstrating the inheritance of an autosomal recessive trait.
The traditional symbol used for a male in pedigrees is a square shape. Females are represented by circles. A horizontal line drawn between the two symbols represents a marriage, with vertical lines dropping down from the couple representing the presence of children. Shapes that are shaded represent individuals that exhibit the studied genetic trait.