Gene interaction

Question 1

How can genes control trait phenotypes?

Answer 1

They can follow simple dominant/recessive relationships

Combine to form completely new phenotypes

Show both dominant and recessive phenotypes simultaneously

Require multiple genes to fully express phenotypes

Question 2

What is nonsense mediated decay?

Answer 2

Protein that has premature stop codon can stop being made completely.

Question 3

In dominant gene disorders what can happen to inheritance?

Answer 3

Heterozygous mutations inherited from affected parent or de novo

Mutation only needs to be on one copy to cause disease

Gain of function mutations can make protein acquire new function

Question 4

In recessive gene disorders what can happen to inheritance?

Answer 4

2 mutations would be needed or compound heterozygous mutations inherited from unaffected parents.

Loss of function mutations are typically what affects these genes resulting in absent or reduced function protein.

Question 5

What are modifier genes?

Answer 5

Genes that interact with other genes and influence their genotypes

Question 6

How are mono and biallelic expression different to each other usually?

Answer 6

Biallelic expression is more prevalent where both alleles of a gene are transcribed

Monoallelic expression is expression where only 1 of the 2 copies of a gene is active.

Question 7

What are the common non-mendelian inheritance patterns?

Answer 7

Multiple alleles

Incomplete dominance

Codominance

Polygenic traits

Pleiotropy

Sex-linked characteristics

Environment also plays an important role in the phenotype

Question 8

What is incomplete dominance?

Answer 8

Phenotype not completely like the dominant character

Question 9

What trait shows incomplete dominance in humans?

Answer 9

Hair texture

Question 10

How is phenotypic ratio different between complete and incomplete dominance?

Answer 10

Incomplete dominance follows genotypic ratio (1:2:1) whereas complete dominance is 3:1 due to dominant gene being most influential

Question 11

What is codominance?

Answer 11

2 Equally dominant alleles expressed concurrently. Eg Blood group (ABO)

Question 12

What are the characteristics of dominance?

Answer 12

Dominance is allelic interaction (result of interaction between genes at the same locus)

Dominance doesn’t alter how genes are inherited but does influence which genes are expressed as a phenotype

Dominance’s allelic interaction is between the products of the genes

Classification of dominance depends on the level which the phenotype is examined

Question 13

What type of expression does sickle cell anaemia show?

Answer 13

Codominance (A/A = wild type, Aa = only sickle under low O2, aa = always sickle)

Question 14

What is a pleiotropic effect?

Answer 14

One gene affects many phenotypic characteristics. eg. cystic fibrosis or sickle cell anaemia.

Question 15

What are polygenic traits?

Answer 15

Multiple genes working together to produce a continuous distribution “Bell Shape” curve.

Most traits follow this pattern

Question 16

What are some examples of polygenic inheritance?

Answer 16

Body type

Height

Skin colour

Hair colour

Eye colour

Intelligence

Question 17

How are X and Y chromosomes different to each other?

Answer 17

X chromosome contains > 1500 genes

Y chromosome has approximately 231 protein coding genes with some that are unique to Y

Question 18

When do sexual differences begin to develop?

Answer 18

At gestational week 7. This is determined by genetic and environmental factors.

Question 19

What is the genotype of turner syndrome, Klinefelter syndrome, and poly-X?

Answer 19

XO = Turner (circle = turn)

XXY = Klinefelter syndrome

XXX = Poly-X females

Question 20

What is the male-determining gene in humans?

Answer 20

Y chromosome gene; SRY gene. It causes gonad to develop into testis.

Other genes on autosomes play an important role.

When testes develop they secrete 2 hormones; testosterone and Mullerian inhibiting hormone (MIH)

Question 21

What does SRY gene control?

Answer 21

Testis formation and in turn the secretion of testosterone and MIH.

Question 22

What are the 2 duct systems present in early embryos and what do they develop into?

Answer 22

Wolffian (male)

Mullerian (female)

At week 7 developmental pathways activate different sets of genes which cause undifferentiated gonads to develop into testes and ovaries which determines gonadal sex of embryo.

Question 23

What is androgen insensitivity syndrome and what is it caused by?

Answer 23

SRY gene is being expressed but receptor for testosterone is defective so they look like females externally with some testes.

Question 24

What is congenital adrenal hyperplasia?

Answer 24

Growth of adrenal cortex resulting in production of lots of androgens. This is a problem in females because of formation of male sex organs

Question 25

What are the features of true hermaphroditism?

Answer 25

Generally have normal chromosomal composition with XX genotype. Appear female but have ovotestis with both spermatagonia and ovarian follicles.

Question 26

What are the features of pseudohermaphroditism?

Answer 26

Males have XY genotype with normal chromosomal composition. Phallic hypoplasia due to insufficient hormone production.

Females have XX genotype with normal chromosome composition but have high levels of androgenic hormones and have masculinised external genitalia.

Question 27

How are post-zygotic mutations distributed?

Answer 27

If cell mutates early in development then it will have descendents in many tissues

Mutation arising later will be more localised.

This results in a mix of cells producing normal and abnormal proteins. Phenotype depends on proportion of cells producing abnormal protein and their distribution.

Question 28

What does mosaicism do to post-zygotic mutation related diseases?

Answer 28

Results in milder disease due to some of the defective genes being switched off.

Question 29

What kind of effects can mosaicism affect?

Answer 29

Can be for chromosomal or single gene disorders

May affect somatic or germline tissues

Can result in a range of abnormalities depending on the amount and distribution of normal cells

Question 30

How does mosaicism apply to Turner syndrome?

Answer 30

Some cells they have are XO but some are XX. Depends on when during development non-disjunction took place.

Question 31

What is skewed X chromosome inactivation?

Answer 31

X-inactivation is not 50/50. Switching off of X chromosomes is often skewed one way or another. (70/30 is considered skewed)

Question 32

What does mosaicism do to mutations?

Answer 32

Can lessen the overall effect.

Question 33

What cells mosaicism?

Answer 33

All clonal descendants of the cell. If germline mutation occurs in mitotic stage prior to meiotic stage then all the descending cells will exhibit the mutation. Somatic cells remain unaffected.

Question 34

What condition is a important example of germline mosaicism?

Answer 34

Osteogenesis imperfecta which is severe in ~6% of people. Mutations in type I collagen genes lead to abnormal collagen, brittle bones, and frequent fractures.