1. Chapter 14
Human Genetics

2. Human Genes and Chromosomes
31 thousand genes located over 46 chromosomes
Autosomes
22 pairs (44 total)
Sex Chromosomes
1 pair (2 total)
XX = female
XY = male
Some genes are located on the sex chromosomes….they are called “sex-linked”

3. Sex Determination: -will a child be male or female?
(XX)
male
(XY)
eggs
sperm Y X X x XX XY X Y

4. Men show sex-linked traits more often…
X-linked recessive traits that are not related to feminine body characteristics are primarily expressed in the observable characteristics, or phenotype , of men.  This is due to the fact that men only have one X chromosome.  Subsequently, genes on that chromosome not coding for gender are usually expressed in the male phenotype even if they are recessive since there are no corresponding genes on the Y chromosome in most cases.  In women, a recessive allele on one X chromosome is often masked in their phenotype by a dominant normal allele on the other.  This explains why women are frequently carriers of X-linked traits but more rarely have them expressed in their own phenotypes.
Why?

5. Sex-linked genes…aka: X-linked genes
Hemophilia
Colorblindness
Duchenne Muscular Dystrophy
**Must take into consideration the X and the Y chromosomes when doing these problems
**We refer to X-linked genes with superscripts
ex: XHXh=female carrier of hemophilia
XhXh=female hemophiliac
Xh Y =male hemophiliac
Why are most of these genes located on the X
chromosome?

6. Completing sex-linked Punnett Squares:

7. Hemophilia
The X chromosome carries genes that help control blood clotting.
In hemophilia, a protein necessary for normal blood clotting is missing.
Hemophiliacs can bleed to death from cuts and may suffer internal bleeding if bruised.

8. Practice Problem:
What are the chances for a mother that is a carrier for hemophilia and a hemophiliac father to have hemophiliac children?

9. Color blindness:
Three human genes associated with color vision are located on the X chromosome.
In males, a defective version of any one of these genes produces colorblindness.
Father
(normal vision)
Normal vision
Colorblind
Male
Female
Daughter
(normal vision)
Son
(normal vision)
Mother (carrier)
Daughter
(carrier)
Son
(colorblind)
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10. Duchenne muscular dystrophy
Duchenne muscular dystrophy is a sex-linked disorder that results in the weakening and loss of skeletal muscle.
It is caused by a defective version of the gene that codes for a muscle protein.

11. Pedigrees and sex-linked traits:
Terms to know:
Normal= unaffected with disease
unshaded in pedigrees
Affected= has disease
fully shaded in pedigrees
Square= male
Circle= female
Hemophilia(heem-o-FILL-ee-ah) is a very rare disease in which your blood clots abnormally. It is an inherited disease, which means that it has to be passed on from another ancestor or relative. It has to do with mostly blood clotting. If a Hemophilia patient bleeds, especially from an injury, they can bleed fairly long and there is a huge loss of blood from the body. Now, platelets need to be injected into the body. As I said before, this is a fairly rare disease, especially for males. "About 1 in 5,000 boys have Hemophilia." (1)Therefore, females aren't as likely to get the gene. People who have Hemophilia have to be extremely careful and cautious about not falling down or even just getting a paper cut.

12. Practice Problem: This pedigree shows the inheritance of hemophilia:
Find the phenotypes for #1, #2, and #4
If #5 marries a woman who is heterozygous for the hemophiliac gene, XHXh, what are their chances for having children with hemophilia?

13. Genetic Disorders: Autosomal
Caused by Recessive Alleles
Recessive disorders only appear if two recessive alleles are present.
Albinism
Cystic fibrosis
Galactosemia

14. Cystic Fibrosis
Sufferers of cystic fibrosis produce a thick, heavy mucus that clogs their lungs and breathing passageways.
The most common allele that causes cystic fibrosis is missing 3 DNA bases.
As a result, the amino acid phenylalanine is missing from the CFTR protein.
Normal CFTR is a chloride ion channel in cell membranes.
Abnormal CFTR cannot be transported to the cell membrane.
The cells in the person’s airways are unable to transport chloride ions.
As a result, the airways become clogged with a thick mucus.

15. Genetic Disorders: Autosomal
Caused by Dominant Alleles :
Dominant disorders are caused by dominant alleles, so only one allele is needed for the disease to show.
Achondroplasia
Huntington’s Disease
Hypercholesterolemia

16. Genetic Disorders: Chromosomal
Caused by Nondisjunction
Down Syndrome—Trisomy Chromosome 21
Down syndrome produces mild to severe mental retardation.
It is characterized by:
increased susceptibility to many diseases
higher frequency of some birth defects
Karyotype: picture of human chromosomes

17. Changes in Chromosome Number usually the result of Nondisjunction
chromosome alignments at metaphase I
n - 1
nondisjunction at anaphase I
alignments at metaphase II
anaphase II

18. Nondisjunction in Sex Chromosomes
Turner’s Syndrome—Monosomy Sex Chromosome (X0)
A female with Turner’s syndrome usually inherits only one X chromosome (karyotype 45,X).
Women with Turner’s syndrome are sterile.
Klinefelter’s Syndrome—Trisomy Sex Chromosome (XXY)
The extra X chromosome interferes with meiosis and usually prevents these individuals from reproducing.

19. Pedigree Analysis Technique to study patterns of human inheritance
Visual representation of family history
Several possible modes of inheritance are able to be identified:
Autosomal dominant traits
Autosomal recessive traits
Sex-linked traits

20. What information do we find in a pedigree?
Horizontal lines between a male and a female = marriage
Vertical lines coming down from a marriage = offspring
Female affected
Male affected
Female with “normal” trait
Male with “normal” trait

21. Autosomal Dominant Appears in both sexes with equal frequency
Can be passed on to the next generation by both males and females
Does not skip a generation
Ex: Huntington’s Disease

22. Autosomal Recessive Appears at equal frequency in both sexes.
Only appears when the affected individual has received one allele from each parent
Skips generations
Ex: Cystic Fibrosis

23. Sex-linked
more frequent in males
trait tends to skip generations

24. MCAS practice question
People who are tune deaf are unable to follow a rhythm. Scientists have evidence that tune deafness can be genetic. The pedigree below traces the inheritance of tune deafness in a family. Individuals in the pedigree are numbered. Scientists have analyzed the inheritance patterns for tune deafness and have concluded that tune deafness is caused by an autosomal dominant allele, T
Provide evidence from the pedigree that shows that the tune deafness allele is autosomal dominant, not autosomal recessive. Explain.
Identify the genotypes of individuals 5 and 6, and then draw the Punnett square for the cross of these two individuals.

25. CASE STUDY: ABO Blood Typing
What inheritance patterns does human blood type demonstrate?

26. ABO Gene
Determined by a single gene with 3 alleles, 2 of which are codominant
Type A Blood (type A marker)
Makes antibody for B
Type B Blood (type B marker)
Makes antibody for A
Type AB Blood (both A & B markers)
Makes no antibodies
Type O Blood (neither marker)
Makes antibodies for A and B

28. Risks of Blood Transfusions
Donor RBCs may not have the same kind of recognition molecules (“markers”) on their surfaces as Recipient RBCs
Potential Result:
Agglutination, a defense response. CLUMPING
Antibodies act against foreign cells and cause them to clump together
Antibodies are produced against antigens NOT present on RBCs

29. When does agglutination occur?
Type A
antibodies ignore A marker, attack B markers
Type B
antibodies ignore B marker, attack A markers
Type AB
antibodies ignore both forms
Type O
antibodies attack both A and B markers

30. Blood Transfusions and Agglutination
Safe Transfusions
Phenotype
(Blood Type)
Antigen on
Red Blood Cell
Genotype To
From
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31. Blood Typing in Forensics
Can be used to determine the blood type of a potential suspect in a crime
Test uses two solutions each containing antibodies to type A and type B antigens.
Solution 1: Anti-A; when mixed with type A blood will cause it to form clumps.
Solution 2: Anti-B; when mixed with type B blood will cause it to form clumps.
If blood clumps under contact with both Anti-A and Anti-B, then it is type AB
O blood does not clump

32. Rh Gene Rh marker determined by a single gene with 2 alleles
Rh+ allele=Dominant=marker present
Rh- allele=recessive=marker not present