Lecture 4: Mendelian Traits in Humans & Pedigree Analysis Flashcards
(40 cards)
1
Q
Albinism
A
- a genetic condition characterized by a deficiency or absence of melanin, the pigment that gives color to skin, hair, and eyes.
- haploinsufficient: biallelic mutations are usually required for the condition to manifest
- straight hairline, no freckles, no hair, round chin, no dimples
2
Q
autosomal inheritance
A
- human autosomal traits are located on the non-sex chromosomes (1-22)
- they may be inherited as autosomal dominant or autosomal recessive
3
Q
autosomal dominant traits
A
- 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 (vertical pattern of transmission)
4
Q
autosomal recessive traits
A
- only homozygous recessive individuals exhibit the affected phenotype
- males and females are equally affected and may transmit the trait
- may skip generations (horizontal pattern of transmission)
5
Q
define a horizontal pattern of transmission
A
where a genetic disorder appears in multiple siblings of the same generation, but not in their parents or ancestors
6
Q
why are pedigrees typically used to study human genetics?
A
humans are not good model organisms: we cannot do controlled breeding experiments on them, so instead use model organisms and pedigrees to dissect mendelian traits of interest
7
Q
define a pedigree
A
an orderly diagram of a family’s relevant genetic features extending through multiple generations
8
Q
what is the purpose of pedigrees?
A
to help us infer if a trait is from a single gene and if the trait is dominant or recessive
9
Q
male
A
10
Q
female
A
11
Q
mating
A
12
Q
parents and children
A
13
Q
dizygotic (nonidentical twins)
A
14
Q
monozygotic (indentical twins)
A
15
Q
sex unspecified
A
16
Q
number of children of sex indicated
A
17
Q
affected individuals
A
18
Q
heterozygotes for autosomal recessive
A
19
Q
carrier of sex-linked recessive
A
20
Q
death
A
21
Q
abortion or stillbirth (unspecified)
A
22
Q
propositus/proband
A
first individual in a family who is identified as having a genetic disorder
23
Q
consanguineous marriage
A
24
Q
draw a graph for Huntington’s Disease
- of all persons carrying the allele, percentage affected with the disease
- age (years)
A
25
Huntington's disease
rare autosomal dominant; onset is later in life, usually after they've had kids.
26
how can we recognise dominant traits in a pedigree diagram?
- affected kids always have at least 1 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
27
give 2 examples of a vertical pattern of transmission
- brachydactyly (rare dominant trait): abnormally short fingers and/or toes, caused by shortened bones in the hands or feet
- polydactyly (rare dominant trait)
28
why don't harmful dominant traits usually persist in populations unless they are late onset?
because of natural selection — specifically, negative selection against traits that reduce survival or reproduction.
29
how can we identify recessive traits in a pedigree?
- 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
30
what is the issue with consanguinity?
- Every person carries several recessive mutations that, if present in two copies, can cause genetic disorders.
- These are usually silent because most people are heterozygous (carrying only one copy).
- Related individuals are more likely to carry the same harmful recessive alleles inherited from a common ancestor.
31
define genetic load
the collection of deleterious recessive alleles present in a population.
32
give two examples of recessive traits
- albinism
- cystic fibrosis
33
what is the purpose of a pedigree with consanguinity (inbreeding)?
frequently uncovers traits that are recessive
34
consanguineous mating can often give rise to
inbreeding depression - offspring that are less fit than their parents
35
genetic counselling sessions
- family history
- pedigree construction
- information provided on specific disorders, modes of inheritance, tests to identify at-risk family members
- testing arranged, discussion of results
- links to support groups, appropriate services
- follow-up contact
36
issues associated with genetic screening
- Why carry out genetic screening at all?
- When is a test accurate and comprehensive enough to
be used as the basis for screening?
- Once an accurate test becomes available at reasonable
cost, should screening become required or optional?
- If a screening program is established, who should be
tested?
- Should private companies and insurance companies
have access to employee and client test results?
- What education needs to be provided regarding test
results?
37
if both parents are heterozygous (Bb) what is the probability that they will produce a BB child?
probability of a sperm with B allele = 1/2
probability of an ovum with B allele = 1/2
probability of a BB child = 1/2 x 1/2 = 1/4
38
parents are heterozygous for a trait, Rr. what is the probability that their child is a heterozygote?
probability of child carrying R from father and r from mother (Rr) = 1/2 x 1/2 = 1/4
probability of child carrying r from father and R from mother (Rr) = 1/2 x 1/2 = 1/4
probability of child carrying Rr = 1/2
39
parents are heterozygous for a trait, Rr. what is the chance that their child carries at least 1 dominant R allele?
probability of child carrying RR = 1/2 x 1/2 = 1/4
probability of child carrying Rr = 1/4 + 1/4 = 1/2
probability of child carrying R _ = 1/4 + 1/2 = 3/4
40
Ellen's brother Michael has cystic fibrosis, an autosomal recessive disease. what is the probability that Ellen's child has a cystic fibrosis-causing allele?
- Ellen and Michael's parents must be heterozygous (if not mentioned in q, assume no disease)
- Probability Ellen is a carrier = 2/3 (not affected thus cannot be ss)
- Probability child inherits cystic fibrosis allele = 1/2
- Probability child carries cystic fibrosis allele from Ellen = 2/3 x 1/2 = 1/3
