Genetics

Question 1

Genotype

Answer 1

the alleles of an organism

Question 2

Phenotype

Answer 2

the characteristics of an organism

Question 3

Dominant allele

Answer 3

an allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state

Question 4

Recessive allele

Answer 4

an allele that only has an effect on the phenotype when present in homozygous state

Question 5

Codominant alleles

Answer 5

pairs of alleles that both affect the phenotype when present in a heterozygote

Question 6

Locus

Answer 6

the particular position on homologous chromosomes of a gene

Question 7

Homozygous

Answer 7

having two identical alleles of a gene

Question 8

Heterozygous

Answer 8

having two different alleles of a gene

Question 9

Carrier

Answer 9

an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele

Question 10

Test cross

Answer 10

testing a suspected heterozygote by crossing it with a known homozygous recessive

Question 11

State that some genes have more than two alleles (multiple alleles).

Answer 11

Some genes have more than two alleles (multiple alleles).

Question 12

Describe ABO blood groups as an example of codominance and multiple alleles.

Answer 12

• three alleles controlling ABO blood groups
• if more than two alleles -> multiple allele
• IA (dominant) – Blood group A
• IB (dominant) – Blood group B
• IA + IB = AB (IAIB)
• ii -> homozygous for I – blood group O
• i = recessive allele
• both allele affect phenotype (dominant)
• Codominant allele = pairs of allele affecting phenotype when present in heterozygote

Question 13

Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans.

Answer 13

• females: XX
• males: XY
• X chromosome is large, containing many genes
• Y chromosome smaller, containing fewer genes
• female always passes on X gene to offspring
• gender of offspring depends on sperm carrying X or Y

Question 14

State that some genes are present on the X chromosome and absent from the Y chromosomes in humans.

Answer 14

Some genes are present on the X chromosome and absent from the Y chromosomes in humans.

Question 15

Define sex linkage.

Answer 15

Sex linkage – when the gene controlling the characteristic is located on the sex chromosome and so we associate the characteristic with gender.

Question 16

State that human female can be homozygous or heterozygous with respect to sex-linked genes.

Answer 16

Human female can be homozygous or heterozygous with respect to sex-linked genes.

Question 17

Explain that female carriers are heterozygous for X-linked recessive alleles.

Answer 17

• females require dominant + recessive allele to be carrier
• inherit recessive from one parent and dominant from other parent
• example: haemophilia

Question 18

Deduce the genotypes and phenotypes of individuals in pedigree charts.

Answer 18

• squares – males
• circles – females
• shaded – affected individuals
• unshaded – unaffected individuals
• if most males affected -> disease is X linked
• if 50/50 -> autosomal
• if dominant -> one parent must have disorder
• if recessive -> neither parent has to have disorder when heterozygous

Question 19

1st stage of meiosis I

Answer 19

Prophase I

• chromosomes coil up tightly, become visible
• homologous chromosomes pair up
• crossing over occurs
• nuclear membrane disintegrates, centrioles travel to poles

Question 20

2nd stage of meiosis I

Answer 20

Metaphase I

• microtubules form spindle
• spindle fibres attach to centromeres of chromosomes
• pairs of homologous chromosomes align on equator

Question 21

3rd stage of meiosis I

Answer 21

Anaphase I

• spindle fibres shorten, pulling homologous chromosomes in opposite directions
• paired homologous chromosomes paired
• pulled to opposite poles

Question 22

4th stage of meiosis I

Answer 22

Telophase I

• nuclear membrane forms around chromosomes at each pole
• chromosomes uncoil
• cell undergoes cytokinesis, forming two daughter cells
• forms two haploid cells
• at end cells either enter short interphase period or proceed to meiosis II
• DNA is not replicated

Question 23

1st stage of meiosis II

Answer 23

Meiosis II
	Prophase II
-	chromosomes coil up again
-	centrioles move to cell poles
-	nuclear membrane disintegrates

Question 24

2nd stage of meiosis II

Answer 24

Metaphase II

• spindle fibres attach to centromeres
• chromosomes align to equator

Question 25

3rd stage of meiosis II

Answer 25

Anaphase II

• spindle fibres shorten
• centromeres split
• chromatids of each chromosome travel to opposite side

Question 26

4th stage of meiosis II

Answer 26

Telophase II

• nuclear membrane forms around chromatids at each pole
• chromatids then called chromosomes
• both cells undergo cytokinesis to form four cells
• chromosomes uncoil
• nuclei form

Question 27

Outline the formation of chiasmata in the process of crossing over.

Answer 27

• in prophase I sister chromatids become tightly linked (synapsis)
• cut made in DNA of one chromatid
• same cut made in a non-sister chromosome
• DNA of chromatid binds to DNA of non-sister chromatid
• paternal and maternal chromosomes no longer tightly linked
• connections between non-sister chromatids remain
• crossing over is finished
  o forming X shaped structure -> chiasmata
• chiasmata links homologous chromosome pairs together
• chiasmata remains until metaphase I

Question 28

Explain how meiosis results in an effectively infinite variety in gametes through crossing over in prophase I and random orientation in metaphase I.

Answer 28

during prophase I
- crossing over
- exchange of parts of non-sister chromatids/homologous chromosomes
- genes on same chromosome are linked
- crossing over allows recombination of (linked) genes
- random orientation of chromosomes
during metaphase I
- independent assortment of genes on different chromosome type
- 2^23 possible combinations of chromosomes
- effectively infinite number of combinations if effects of crossing over included

Question 29

State Mendel’s law of independent assortment.

Answer 29

Allele pairs separate independently during gamete formation which means that the transmission of traits to offspring are independent to one another.

Question 30

Explain the relationship between Mendel’s law of independent assortment and meiosis.

Answer 30

during metaphase I

• homologous pairs of chromosomes align along equator
• orientation of chromosomes is random
• -> during anaphase I either chromosome moves to one pole
• depending on which way pair is facing
• -> independent orientation, forming basis of Mendel’s law
• unlinked genes found on different chromosomes
• when homologous pairs separate, allows formation of daughter cells with random assortment of chromosomes and alleles

Question 31

Distinguish between autosomes and sex chromosomes.

Answer 31

• sex chromosome determine gender (XY)
• autosomal chromosomes = remaining chromosomes
• 22 autosomal pairs in humans
• 23 pairs in total

Question 32

Define linkage group.

Answer 32

A pair or set of genes on a chromosome which tend to be inherited together.