HMB265H1 Lecture Notes - Lecture 4: Melanin, Albinism, Consanguinity

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emeraldcrow840

20 May 2018

School

UTSG

Department

Human Biology

Course

HMB265H1

Professor

Maria Papaconstantinou

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Lecture 4: Mendelian Traits in Humans & Human Pedigree Analysis

Mendelian inheritance and humans: same principles apply

• Albinism: autosomal recessive condition

o Two copies of A produce Tyrosine enzyme → melanin

▪ Aa is also sufficient to produce melanin

▪ aa no tyrosine → albino

Autosomal Inheritance

• Human autosomal traits are located on the non-sex chromosomes (1-22)

• They may be inherited as:

o Autosomal dominant

o Autosomal recessive

• Homozygous dominant and heterozygotes exhibit the

affected phenotype

• Males and females are equally affected and may

transmit the trait

• Affected phenotype does not skip a generation

Autosomal Recessive

• Only homozygous recessive individuals exhibit the affected phenotype

• Males and females are equally affected and may transmit the trait

• May skip generations

• 2 recessive alleles are needed for the phenotype to be expressed

Pedigrees

• Cannot do controlled breeding experiments on humans, use model organisms and human

pedigrees to dissect Mendelian traits of interest

o Humans have a very long generation time

• Pedigrees are an orderly diagram of a family’s relevant genetic features extending through multiple generations

• Pedigrees help us infer it a trait is from a single gene and if the trait is dominant or recessive, autosomal or sex-

linked

Dominant Traits

• Affected children always have at least one affected parent

• As a result, dominant traits show a vertical pattern of inheritance: the trait shows up in every generation

• Two affected parents can produce unaffected children, if both parents are heterozygotes

• Vertical pattern of inheritance in pedigrees

o Affected individual in every generation

Recessive Traits

• Affected individuals can be the children of two unaffected carriers, particularly as the result of consanguineous

matings

• All the children of two affected parents should be affected

• Rare recessive traits show a horizontal pattern of inheritance: the trait first appears among several members of one

generation and is not seen in earlier generations

• Recessive traits may show a vertical pattern of inheritance if the trait is extremely common in the population

• Most traits are recessive

• Dominant deleterious traits do not continue to manifest

• Gene load→lethal recessive alleles

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Related Questions

13. The arms of a human and the wing of a bat are _________________ structures, composed of the same bones that have been modified for different functions.

analogous

vestigial

c.	homologous

d.	convergent

14. Xeroderma pigmentosum is an autosomal recessive genetic disorder. People who are homozygous for the disease-causing allele have skin that is damaged by exposure to ultraviolet radiation; this can lead to skin cancer at an early age. The mutation that causes this disorder is in a gene that functions:

a.	to repair DNA by excising (removing) nucleotides damaged by UV light.

b.	in lysosomes, to break down lipids that otherwise accumulate in the nervous system.

c.	to produce the UV blocking pigment melanin in the skin cells

d.	in red blood cells to carry oxygen to the skin.

15. The Amish are a religious sect descended from a small group of colonists who came to the United States over 200 years ago. Their religious beliefs have kept them isolated from society. The Amish have a much higher than average incidence of both polydactyly (extra fingers and toes) and dwarfism in their communities. This is most likely explained by:

a.	inheritance of acquired characters.

b.	the founder effect.

c.	disruptive selection.

d.	natural selection.

16. In the structure of DNA that Watson and Crick proposed, the sides of the "ladder" of the DNA molecule are always the same distance apart, making the molecule a constant width. This occurs because when the bases hydrogen bond to form the rungs of the "ladder", a:

a.	purine always pairs with a pyrimidine.

b.	purine always pairs with a purine.

c.	pyrimidine always pairs with a pyrmidine.

d.	The structure of the double helix is not determined by how the bases pair.

17. The RNA transcript of DNA that travels to the cytoplasm, carrying the instructions to make a protein, is called:

rRNA.

tRNA.

mRNA.

d.	RNA polymerase.

18. When DNA is replicated, the error rate is approximately one error for every 10,000 nucleotides copied. However, that error rate is reduced to only 1 error for every 1 billion nucleotides. The DNA sequence is "corrected" by:

a.	repeating S phase to see if the error rate is lower the second time around.

b.	enzymes that proofread the DNA and repair errors.

c.	messenger RNA during the process of transcription.

d.	Any errors made during DNA replication cannot be corrected. If errors are detected after S phase, the cell is destroyed.

19. Mendel's principle of segregation says that:

a.	when gametes are formed, each gamete receives only one allele for a particular gene.

b.	some genes are dominant to others.

c.	a testcross must be used to determine the genotype of an organism with a dominant phenotype.

d.	all of the above

20. Tay-Sachs disease is lethal before reproductive age, but the allele persists in Louisiana French Canadians because:

a.	it is a dominant allele.

b.	new mutations causing this disease are common in that population.

c.	it is not expressed in the phenotype of heterozygous individuals.

d.	the disease is contagious and can be transmitted by nonheritable means.

21. Physical features that are similar in organisms which are not closely related to each other are most likely a result of ____________. A good example of this is the similarities seen between some marsupial and placental mammals.

a.	natural selection.

b.	adaptation to similar environments

c.	convergent evolution

d.	All of the above phrases can be used to correctly fill this blank.

22. In humans, which of the following sex chromosome compliments could be found in males?

23. The proteins that are associated with eukaryotic chromosomes:

a.	help to package the DNA within the nucleus.

b.	are involved in gene expression, through binding with the DNA.

c.	may be replicating or transcribing the DNA.

d.	All of these are true of proteins that are associated with eukaryotic chromosomes.

24. A sequence of DNA nucleotides that contains the information to produce a single protein is a(n):

codon.

gene.

c.	polypeptide.

d.	anticodon.

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HMB265H1 Lecture Notes - Lecture 4: Melanin, Albinism, Consanguinity

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