1. Complex & Human Genetics
Chapter 14

2. Chromosomes & Phenotype
Take a look around the room. What are some traits that vary from person to person?
About 99.9% of our DNA is identical to everyone else’s DNA.
How can there be so much variation with only 0.1% of our DNA being different?

3. Chromosomes & Phenotype
The answer lies in the fact that even though many human genes are complete dominant or recessive, there are also many that are not, and even many that are just one factor in determining a phenotype
Many human traits have numerous genes or many alleles possible

4. Chromosomes & Phenotype
Most genes for humans are found on the autosomes and are inherited simply – just like Mendel predicted
Many human disorders are recessive (luckily) & autosomal (not on sex chromosomes)

5. Chromosomes & Phenotype
When a disease is recessive, a person who is heterozygous for the disease will not have the disease but is said to be a carrier of the disease
They can pass on the allele to their offspring

6. Chromosomes & Phenotype
If two people who are both carriers have children, it is possible for their children to have the disorder
Cystic Fibrosis is an example of a recessive human genetic disorder

7. Chromosomes & Phenotype
Suppose a carrier of cystic fibrosis marries a healthy non-carrier.
What are the genotypes of each individual?
Can any of their children have cystic fibrosis?

8. Remember, cystic fibrosis is a recessive disorder
A carrier is always heterozygous, so let’s say the dad is Ff.
A healthy non-carrier is homozygous dominant, so the mom would be FF.

9. Let’s use a Punnett Square to determine the possible outcomeF f F FF Ff Ff F FF

10. None of their children will have the disorder, but what percent are likely to be carriers of the disorder?

11. Dominant Disorders Some diseases are not recessive
Much less common than recessive disorders
Only 1 allele is needed to have the disease
Huntingdon’s Disease

12. Huntingdon’s Disease
A degenerative disorder that appears in adulthood (usually AFTER the person is of childbearing age)
Always fatal
The brain degenerates slowly & the person loses control of everything

13. Huntingdon’s Disease
Often, the affected individual already has children
The children will have a 50% chance of having the disease as well! (assuming the affected parent is heterozygous)

14. Why would it be unlikely that a fatal dominant disease causes death in early childhood or before puberty?

15. Sex-Linked Diseases
Some human disorders are NOT found on the autosomes, but are instead located on the X chromosome
Remember, females have 2 X’s (XX) and males have 1 X & 1 Y (XY).

16. Mothers always pass one of their X’s to every one of their kids – boy or girl
Fathers pass an X to their daughters and a Y to their sons
Genes on the X chromosome follow the same rules in females as on autosomes

17. However, because males have only 1 X, only 1 copy of a recessive gene is needed to have a disorder
Color-blindness is an example of a recessive sex-linked disorder

18. If a color blind male marries a normal female, what are the chances their sons will be color blind? Their daughters?
Let’s use a Punnett square, but this time we need to include the sex chromosomes
Dad = XbY
Mom = XBXB

19. What percent of their daughters will be carriers of color blindness?
What percent of their sons will be carriers of color blindness?
What percent of their daughters will be normal?
What percent of their sons will be normal?

20. In lab today, we will be modeling the inheritance pattern of a sex-linked gene (color blindness) with coins
Turn to page 202 in a textbook and read the Quick Lab.
Each group will have a different cross to model, two coins and tape.

21. What percent of the children (daughters and sons) will be color blind if a normal man marries a color blind woman?
What percent of the children will be color blind if a color blind man marries a carrier woman?

22. What % of the children will be color blind if a normal man marries a carrier woman?
A couple has 4 children. The 2 girls are color blind. One son is color blind, but the other has normal vision. What are the genotypes of the parents?
If they have another baby, what are the odds it will NOT be color blind?

23. Not all complex traits are determined by sex-linked genes, in fact, most are autosomal
As we saw in chp 6, some traits can show incomplete dominance or co-dominance
Incomplete dominance is a blending of traits to produce a third distinct trait

24. Many traits in flowers and other animals show incomplete dominance – the color of betta fish (green GG, steel blue BB, or royal blue BG)
Many flowers can be red, white or pink due to incomplete dominance

25. Co-dominance is when 2 traits are fully and separately expressed
In humans, blood type exhibits co-dominance for the A and B alleles.
Neither A nor B is recessive, so if you have both alleles, you are blood type AB.

26. Human blood type is also an example of multiple alleles because there is a third possible allele – O – which is recessive to both A and B

27. Sometimes, many genes interact to determine a phenotype
Skin color & eye color in humans are examples of polygenic traits
This allows for a range of phenotypes

28. Other times, one gene can override all the others for a trait – this is called epistasis
Fur color in many animals is determined by numerous genes, and 1 gene can block the expression of all the others – making the animal albino – Humans can be albino too!

29. Albino Animals

30. Genes are not the only thing that can determine phenotype – oftentimes the environment plays a role as well
Height, weight, personality, disease susceptibility and other traits are all determined by a mix of genes and environment to some degree

31. How do we trace Genes in a Family?
Sometimes, it is useful to look at an entire family to determine the chances certain offspring have of being born with a disease
Geneticists can create a “family tree” of a couple to help them decide if they should have children or not

32. How do we trace Genes in a Family?
Because many genetic disorders are recessive, a healthy couple usually cannot know for sure if they are carriers and can pass the disorder to their children
A genetic counselor will make their family tree – called a pedigree

33. How do we trace Genes in a Family?
A pedigree is a pictorial representation of a family, showing the genotypes and/or phenotypes of the people in the family in relation to a single trait

34. Pedigrees
A pedigree can be made for any trait that is inherited simply – i.e. NOT a trait that is determined by numerous genes or interaction with the environment, such as skin color, height or weight

35. Pedigrees Symbols

36. Scenario #1
Jim and Susan get married and have 4 children – Marty, Sarah, Timmy and Jill. Marty marries Wendy and has a son, Jack. Jill marries Mike and has twin girls, Donna and Dottie. Dottie marries Joey and they have 3 daughters – Mary, Minnie and Molly.
Draw a pedigree if Jim, Susan and all their children, Donna and Minnie all show the trait in question.
Is this trait dominant or recessive? How do you know?
Is it sex-linked or autosomal? How do you know?

37. Scenario #2
Look at the following pedigree and determine the genotypes of all the individuals, if possible.
Determine if the trait is autosomal or sex-linked, dominant or recessive.

38. Karyotypes
A karyotype is a picture of the actual chromosomes in a cell
A karyotype can show if a person has any chromosomes missing (or extra), or if there is a mistake (deletion or insertion) in one of the chromosomes

39. Normal Female

40. Normal Male

41. Karyotypes
Someone who has Down syndrome will have a karyotype that looks like this:
What is different?
Is this person male or female?

42. Karyotypes
Someone who has Turner’s syndrome will have a karyotype that looks like this:
What is different?
Is this person male or female?

43. Karyotypes
Turner’s syndrome patients are always female and are mentally retarded.
They have only one X chromosome

44. Karyotypes Here is another one: What is different?
Is this person male or female?