Mendelian genetics

Question 1

What was the first applied genetic practice?

Answer 1

Artificial selection of new varieties. Led to very quick evolution of maize into a larger size from the wild ancestor, teosinte. The same thing also occurred with the domestication of wolves into many hundreds of dog breeds.

Question 2

What is the study of genetics?

Answer 2

The study of how phenotypes are inherited based on observations of:

• resemblance between parents and offspring
• diversity within sibships (group of offspring with same parents), litters or breeds

Question 3

Misconceptions of Mendel’s time?

Answer 3

1. One parent contributes more to an offspring’s inherited traits. E.g. Aristotle believed it to be the male and that a fully formed homunculus was inside the sperm
2. Blended inheritance – the traits of the parents are blended in their offspring and forever changed.

Question 4

Difference between Watson and Crick, and Mendel?

Answer 4

Watson and Crick showed genes as units of DNA.

Mendel used the concept of genes as units of inheritance.

Question 5

Define monogenic trait?

Answer 5

A trait which is discrete and due to differences in the product of a single gene.

These are special cases as normally very large numbers of genes interact to make up the phenotype.

Question 6

How many genes contribute to human height?

Answer 6

Over 700

Question 7

The types of mutation?

Answer 7

Loss or gain of function

Question 8

Describe loss of function?

Answer 8

There are many places in a gene where a mutation could knock out the function.

Often cause recessive traits – when a wildtype is given by presence of a protein but isn’t sensitive to quantity, dominant homozygous or heterozygous result in wildtype phenotype

Can cause dominant traits where the phenotype is sensitive to the quantity of protein

Question 9

Example of dominant loss of function?

Answer 9

Brittle bone disease, type 1 osteogenesis imperfecta in heterozygotes.

Caused by a loss of function in the gene encoding type 1 collagen. There is a threshold of protein which is required.

Question 10

Describe gain of function?

Answer 10

Occasionally the mutation can lead to a change in functionality, maybe by producing a different substrate.

Dominant traits are rare. Presence of any of the mutant proteins affects expression of the phenotype.

Recessive traits such as sickle cell anaemia – deforms red blood cells when oxygen is released. Those who are heterozygous have better resistance to malaria but don’t exhibit sickle cell anaemia.

Question 11

What is the wildtype allele?

Answer 11

The most common phenotype in a natural population.

Question 12

Define multiple allele genes?

Answer 12

ABO blood groups have three alleles – IA, IB, I. A specifies an enzyme that adds sugar A, etc, whilst I doesn’t produce a functional sugar adding enzyme.

Question 13

Define co dominant genes?

Answer 13

Hybrid may resemble both parents in that both traits are visible – AB blood group.

Question 14

Define incomplete dominance?

Answer 14

Hybrid is a result of phenotype blending, will resemble the middle ground between the parents – cross red and white snapdragons to get pink offspring.

Question 15

Define pleiotropy?

Answer 15

One gene may contribute to several visible characteristics. Multiple phenotypes which are possibly distinct and seemingly unrelated, can be due to allelic variation in a single gene.

Question 16

Example of pleiotropy?

Answer 16

Nail-patella syndrome can affect thumbnails, kneecaps and also possible glaucoma.

Question 17

Define lethal alleles?

Answer 17

Inheritance of a recessive allele that produces a dominant coat colour phenotype which can also give incorrect ratios, etc.

Question 18

Define variable penetrance?

Answer 18

Enzyme variants can be temperature sensitive so only function at cooler body temperatures. Siamese cats which are homozygous for a particular allelic variant of a gene that codes an enzyme catalysing melanin.

Question 19

Outline the main characteristics of cystic fibrosis?

Answer 19

~caused by a single gene on chromosome 7

~CFTR (Cystic Fibrosis Transmembrane Conductance Regulator Protein) is involved in production of sweat, digestive fluids and mucus. Secretions become thicker when mutated

~causes several symptoms (pleiotropy) – lung infection, male infertility, poor growth, abnormal mucus in lungs, sweat glands produce more chloride/sodium, issues with gastrointestinal/reproductive tracts.

~recessive trait

~multiple alleles - more than 1000 mutations in the gene have been identified in those with cystic fibrosis, but delta F508 accounts for 70% of mutations and causes the abnormal channel to break down shortly after it’s made so never transports chloride ions across the membrane

~variable penetrance – features of disorder and their severity varies among affected individuals

Question 20

What characteristics does a good model organism for genetic analysis have?

Answer 20

• short generation time
• can be inbred
• simple reproductive biology
• small size
• large numbers of progeny so mathematical patterns are easier to find

Question 21

Why was Mendel’s organism a good one to choose?

Answer 21

The garden pea – can grow lots in a very small space. There are some clear cut discrete traits (antagonistic pairs) which are easily distinguished from each other.

Question 22

Mendel’s observations?

Answer 22

The mating of parents with antagonistic (pure bred) traits produces hybrids. The trait in an antagonistic pair that was shown in the hybrid’s phenotype is known as dominant.

The dominance of the trait was independent of the parent.

Reappearance of traits in F2 generation disproves the blending theory.

Each individual carries two copies of each gene – one from each parent. Segregate during gamete formation.

Alleles unite at random, one from each parent at fertilisation.

Dihybrid crosses reveal Mendel’s Law of Independent Assortment – the segregation of alleles at one locus is independent of the segregation of alleles of another locus.

Question 23

How can Mendel’s laws be applied to paternity tests?

Answer 23

Markers have multiple alleles – highly variable Short Tandem Repeats (microsatellites).

These are formed by slippage during replication. One repeat gets deleted if this occurs on the copied strand, whilst one repeat gets inserted if this occurs on the copying strand.

Can work out the probability that the allele could have been inherited by anyone in the population.

~Find the probability of the two events:

1. Frequency of allele in population – 0.15
2. Probability of father producing that allele 0.5
3. 0.5/0.15 = 3.3333

Evidence at multiple loci combined to reach a high confidence in assigning paternity.

Question 24

What happened after Mendel’s death?

Answer 24

1902 – Chromosomal basis of heredity: Based on microscopic observations of segregation of homologous chromosome pairs during gamete formation. Chromosomes parallel the behaviours of Mendel’s units of inheritance so it was inferred they carried the genetic material

1907 – showed this was the basis of why hybrid flowers were “unstable”, was commissioned by the Royal Horticultural Society and led to coining of words “genetical” and “genetics”

Question 25

What is the phenotypic gambit?

Answer 25

The phenotypic gambit refers to the simplifying assumption that complex traits, such as behavioural traits, can be modelled as if they were controlled by single distinct alleles, representing alternate strategies.