Inheritance

Question 1

What are the features of autosomal recessive inheritance?

Answer 1

Expressed rarely, as you need to have two recessive alleles

Usually skips generations

Affects both males and females in equal number, as its not sex linked

Requires the affected individual to be homozygous recessive in order to express

A heterozygous individual is classified as a carrier

Can result from two unaffected parents who are carriers

Question 2

What are the features of autosomal dominant inheritance?

Answer 2

Affects both males and females in equal number, as its not sex linked

Means anybody affected will have an affected parent, as they must have revived a dominant alleles from them

If the parent do not show a disorder no further generation will have the disorder either

Results in non-affected individuals being homozougoue recessive

Anybody affected is either double dominant or heterozygous

Question 3

What are the features of sex-linked recessive disorders?

Answer 3

Affects more males than females, males only have one X so there’s a greater chance that it will affect them

Not passed on from an affected male to son, as the male would have given his Y chromosome

Needs an individual to be homozygous if they are female

Will be expressed in a male who has one copy of the defective allele

Will not be expressed in a female or male who is normal homozygous

Will not be expressed in heterozygous females

Means that usually the mother of an affected male is unaffected herself

Means that all daughters with affected father will either be affected or be carriers

Means that the daughters who are affected must have a father which is affected and a mother who is at least a carrier

Question 4

What do squares represent in pedigree charts?

Answer 4

Males

Question 5

What do circles represent in pedigree charts?

Answer 5

Females

Question 6

Name the six mechanisms of non-mendelian inheritance

Answer 6

Sporadic cases

Incomplete penetrance

Genomic imprinting

Anticipation

Mitochondrial Inheritance

Multigenic Inheritance

Question 7

How do sporadic cases result in non-mendelian inheritance?

Answer 7

They result in recurrent de novo mutations

Question 8

How does incomplete penetrance results in non-mendelian inheritance?

Answer 8

This modifies genes and is an environmental trigger

Question 9

What is genomic imprinting?

Answer 9

When only one of the two copies inherited from your parents are active. Which copy is active depends on the parent of origin. In other words, gene expression is parent-of-origin dependent.

Question 10

How does inheriting two maternal genomes effect an embryo? What are these zygotes called?

Answer 10

Poor membrane and placenta development

Abnormal mass of embryonic tissues, which may develop into ovarian teratomas

Gynogenotes

Question 11

How does inheriting two paternal genomes effect an embryo? What are these zygotes called?

Answer 11

Normal membrane and placenta development but will be abnormal

Mass of placental structures and a mole forming in the mother

Adrogenotes

Question 12

How does genomic imprinting result in non-mendelian inherticane?

Answer 12

If a mother is a carrier of an autosomal recessive disorder, they can still pass on the disorder to their offspring if they pass on two mutated copies of the chromosome.

Question 13

What is anticipation?

Answer 13

A triplet expansion of DNA. This trinucleotide repeat is a sequence of three DNA nucleotides that is repeated a number of times in a row.

Question 14

How does anticipation result in non-mendelian inheritance?

Answer 14

DNA segments with an abnormal number of these repeats are unstable and prone to errors during cell division. The number of repeats can change as the gene is passed from parent to child.

Question 15

What is mitochondrial inheritance?

Answer 15

Individual inherits a trait which is encoded in the mitochondrial genome

Individuals with a mitochondrial disease may be male or female but they are all inherited maternally. This is because mitochondrial DNA is only passed from mother to offspring. This means no male with the disease can transmit it to his children

Question 16

What is mitochondrial DNA? How does it compare to normal DNA?

Answer 16

Each mitochondrion is made up of DNA and is referred to as mtDNA.

Much smaller
Chromosomes are round shaped
Chromosomes are not diploid but polyploidy - thousands of copies of mitochondrial genes

Question 17

What are homoplasmy cells?

Answer 17

Cells that contain identical mtDNA

Question 18

What are heteroplasty cells?

Answer 18

Cells that contain a mixture of two populations of mtDNA, mutant and wild type

Question 19

How does mitochondrial disease vary within families? How?

Answer 19

Heteroplasmy

This is because heteroplasmy means that an individual who has a mitochondrial disease that only affects on mitochondrion in each of its cells can produce offspring how can have multiple affected mitochondrion. This can continue throughout several generations, where the number of affected mitochondria continue to increase. The more affected mitochondria, the more affected the individual is by the disease.

Question 20

How do mtDNA mutations occur?

Answer 20

Lack of DNA repair system

Lack of protective proteins

Inner mitochondrial membrane producing oxygen free radicals during oxidative phosphorylation

Question 21

What is polygenic inheritance?

Answer 21

Occurs when one characteristic is controlled by two or more genes. Often genes are large in quantity but small in effect, with each of the genes having an equal influence and each of the alleles having an additive effect on the phenotype outcome

Question 22

What is fitness?

Answer 22

The relative ability of organisms to survive long enough to pass on their genes

Question 23

What affects fitness?

Answer 23

Alleles - if deleterious, can decrease fitness but if advantageous, can increase fitness. If selective pressure change, the importance of different alleles may change

Social ability - increase their ability to defend themselves, find food and find a sequel partner

Question 24

What diseases are commonly due de novo recessive mutations?

Answer 24

Sickle cell disease

Thalassaemia

Question 25

What are the two ways that we can inherit dominant and X-linked genes?

Answer 25

Inherited

De novo

Question 26

How do we work the genotype freuqency?

Answer 26

Dividing the number of individuals with that genotype by the total number of individuals

Question 27

How do we work the allele freuqency?

Answer 27

Adding up all the number of individuals who have that allele and multiplying it by how many alleles that they have. We divide this number by the total number of alleles

Question 28

What is always in a one to one ratio from first to second generation? What does this mean?

Answer 28

The relative frequencies of p (recessive) and q (dominant) remain constant in both the first and second generation

Dominant alleles do not become more common at the expense of recessive ones, when there are no selection pressures. Due to the allele frequencies remaining constant generation to generation, the relative proportion of genotype frequencies also remain constant.

Question 29

What are the six assumptions we make when using the hardy-weinberg equation?

Answer 29

There are no new mutations

Migration is negligible - as migrations results in the introduction of new alleles into a different population. Intermarriage of individual results in a hybrid population

Mating is random - non-random mating leads to an increase in mutant allele.

There are no selection pressures - as this would result in natural selection, where biological traits become less or more common in a population

Population size is large - balance out fluctuations, whereas small populations exhibit genetic drift and founder effects

Allele frequencies are the same in both sexes

Question 30

What is assortative mating?

Answer 30

When individuals chose their partners based on shared characteristics

Question 31

What is negative selection?

Answer 31

It reduces an individuals reproductive fitness, decreasing the prevalence of traits and leads to the gradual reduction of mutant alleles

Question 32

What is positive selection?

Answer 32

It increases an individuals reproductive fitness, increase the prevalence of adaptive traits and leads to heterozygote advantage

Question 33

What is genetic drift?

Answer 33

Random fluctuation of one allele transmitted to a high proportion of offspring by chance.

It is different from natural selection as the change in gene frequencies are due to chance or random events in the same generation rather than over successive generations.

Question 34

What is the founder effect?

Answer 34

The reduction in genetic variation that results from a small subset of a large population establishing a new colony

Question 35

How do we work out the carrier frequency?

Answer 35

We work out the mutant allele frequency by dividing the number of affected individuals by the total number of individuals. (q)

We the work out the normal allele using the fact that p + q = 1. This is because we know q, and we can work out p by plugging into the equation and rearranging it.

We then plug the values into 2pq, which is part of the Hardy-Weinberg equation

Question 36

What is microarray genetic test?

Answer 36

The chromosome patterns are assessed.

Question 37

What are the advantages of next-generation sequencing? What are the disadvantages of next-generation sequencing?

Answer 37

Reduces the cost

Reduces the time to interpret the sequence data - as a computer does it instead of a human

Interpretation of variants takes just as long and needs skilled scientists

Question 38

What are the five classes of variations?

Answer 38

Class One – Clearly not pathogenic

Class Two – Unlikely to be pathogenic

Class Three – Unknown significance (VUS), which means that we can’t prove they are the cause of the disease but know they make some sort of difference

Class Four – Likely to be pathogenic

Class Five – Clearly pathogenic

Question 39

What is trio-based analysis? What are the advantages? What are the disadvantages?

Answer 39

When we genetically test both the parents and the child.

Advantages - reduces false calls

Disadvantages - costs more