Semester 2 Week 1

Question 1

What is the locus?

Answer 1

The locus is the specific location of each gene on a chromosome.

Question 2

What is an allele?

Answer 2

An allele is a different version of the same gene, that is found on the same position on
a chromosome and codes for the same character (trait).

Question 3

What does homozygous mean?

Answer 3

Homozygous means that an organism has 2 identical alleles for a character (trait).

Question 4

What does heterozygous mean?

Answer 4

Heterozygous means that an organism has two different alleles for a character (trait).

Question 5

What are homologous chromosomes?

Answer 5

Homologous chromosomes have the same length and shape and they carry genes controlling for the same inherited characters.

Question 6

What is the phenotype?

Answer 6

The phenotype is the physical appearance of an organism

Question 7

What is the genotype?

Answer 7

The genotype is the genetic make-up of an organism.

Question 8

What is a dominant allele?

Answer 8

A dominant allele is a variation of a gene that produces the same trait in phenotype regardless of what the other allele is.

Question 9

What is a recessive allele?

Answer 9

A recessive allele doesn’t produce a phenotype when there’s a dominant allele. Both alleles need to be recessive for the trait to show in phenotype.

Question 10

What is the law of segregation?

Answer 10

The law of segregation states that the two alleles (versions) of a gene separate during gamete formation (meiosis) and end up in different gametes (sex cells).

Question 11

What is the law of independent assortment?

Answer 11

The law of independent assortment states that during the formation of gametes, different pairs of genes segregate independently of each other.

In other words, the inheritance of one gene doesn’t influence the inheritance of another gene.

Question 12

REMEMBER - If 2 genes are located on the SAME chromosome, they will (probably) be inherited together, so law of independent assortment DOESN’T apply

Answer 12

HOWEVER - If 2 genes are located on different chromosomes, they will be inherited independently, so law of independent assortment DOES apply.

Question 13

What are the 3 scenarios where the inheritance of characters by a single gene deviates from simple Mendelian patterns?

Answer 13

3 scenarios:
1) When the alleles aren’t completely dominant or recessive
2) When a gene has 2 or more alleles (alternate version of same gene)
3) When a gene produces multiple phenotypes.

Question 14

What is complete dominance?

Answer 14

Complete dominance is when the phenotypes of heterozygote and dominant homozygote are identical (dominant allele expressed)

Question 15

What is incomplete dominance?

Answer 15

Incomplete dominance is when phenotype of heterozygotes is somewhere between the phenotypes of 2 parents.

Question 16

What is codominance?

Answer 16

Codominance is when 2 dominant alleles both affect the phenotype in separate ways (both alleles are expressed).

Question 17

Explain how human blood group type is an example of codominance

Answer 17

This is why human blood group type is an example of codominance:

A and B - dominant alleles
O - recessive allele
AA and AO genotype - Blood group A
BB and BO genotype - Blood group B
OO genotype - Blood group O
AB genotype - Blood group AB (both dominant alleles expressed).

Question 18

What is pleiotropy?

Answer 18

Pleiotropy is the property of most genes to have multiple phenotypic effects.

Pleotropic alleles are responsible for multiple symptoms of hereditary diseases like cystic fibrosis and sickle cell disease.

Question 19

What is epistasis?

Answer 19

Epistasis is when the effects of one gene are modified by other genes

Question 20

What is polygenic inheritance?

Answer 20

Polygenic inheritance is when 2 or more genes control the expression of a single phenotype.

Question 21

How is albinism an example of epistasis?

Answer 21

Albinism is a recessive disorder where there is a partial or complete lack of pigment in skin, hair or eyes. This is caused by absence or defect of tyrosinase, enzyme responsible for production of melanin.

Albinism allele inhibits expression of genes responsible for hair and eye colour, so it’s example of epistasis.

Question 22

What are examples of internal and external conditions that can affect phenotypes?

Answer 22

Some internal and external conditions that affect phenotype are:
Temperature
Chemicals
Nutritional habits

Question 23

What are multifactorial characters?

Answer 23

Multifactorial characters are characters that are influenced by both genetic and environmental factors, such as height.

Question 24

What is a karyotype?

Answer 24

A karyotype is an ordered display of the pairs of chromosomes from a cell.

Question 25

What is a human somatic cell?

Answer 25

A human somatic cell is any cell that isn’t a gamete (sex cell). They have 23 pairs of chromosomes (they are diploid).

Question 26

What are autosomes?

Answer 26

Autosomes are chromosomes that don’t determine the sex of the organism.

The human karyotype has 22 pairs of autosomes and 1 pair of sex chromosomes.

Question 27

What are the three types of inherited human disorders?

Answer 27

The three types of inherited human disorders are:
Autosomal dominant disorders (e.g Huntington’s)

Autosomal recessive disorders MOST COMMON (e.g Albinism)

Sex-chromosome linked disorders - CAN BE DOMINANT OR RECESSIVE (e.g haemophilia or colour blindness)

Question 28

REMEMBER - Heterozygous individuals who carry recessive allele for a disorder but are phenotypically normal are called CARRIERS.

Answer 28

SO if a recessive allele that causes a disease is rare, then the chances of 2 carriers meeting and mating is low.

However, consanguineous matings (matings between close relatives) increase the chance of mating between two carriers of same allele.

Question 29

What is prenatal screening?

Answer 29

Prenatal screening is when a foetus or embryo is tested for diseases or conditions. It can detect birth defects such as:

Chromosomal abnormalities (e.g. Down’s syndrome) by karyotyping

Genetic disorders (e.g. sickle cell anaemia) by genetic testing