Level 2 Bio: Monohybrid Crosses, Dihybrid Crosses, Dominance, Multiple & Lethal Alleles

Question 1

What are the 3 types of dominance

Answer 1

1) Complete dominance
2) Incomplete dominance
3) Codominance

Question 2

What is incomplete dominance

Answer 2

Incomplete dominance occurs when neither allele dominates the other. When both alleles are present in the heterozygous genotype, both contribute to produce the phenotype that is intermediate or a blend of genetic information.
Three phenotypes may occur compared to two with complete dominance.

Question 3

How many phenotypes can you get with incomplete dominance?

Answer 3

Three

Question 4

What would you get when crossing a red homozygous flower with a white homozygous flower if there was incomplete dominance

Answer 4

A pink flower.

Question 5

What would you get if you crossed 2 heterozygous pink flowers (incomplete dominance resulted in pink)

Answer 5

25% red, 50% pink, 25% white

Question 6

What is Co-dominance

Answer 6

When a single gene has more than one dominant allele. When an individual is heterozygous, they express both phenotypes associated with both alleles. All the alleles are given Capital letters to show that they are co-dominant.

Question 7

What do you do to the way alleles are written if they are co-dominant

Answer 7

Give co-dominant alleles Capital letters.

Question 8

What is complete dominance

Answer 8

When one allele is dominant over a recessive allele (the normal one)

Question 9

What is multiple alleles

Answer 9

Where genes have more than 2 alleles.

Question 10

What do multiple alleles always do (2 things)

Answer 10

1) Occupy the same locus on the chromosome.
2) They always influence the same characteristic.

Question 11

What is lethal alleles (Where not all offspring survive)

Answer 11

This is a mutation of a gene that causes one genotype to die.

Question 12

What is possible to do with lethal alleles?

Answer 12

Dominant lethal alleles are possible but usually eliminated as their expression leads to death.

Question 13

What is a fact about recessive alleles to do with lethal alleles

Answer 13

Recessive lethal alleles are only fatal to the homozygous recessive as they are masked otherwise by the dominant allele.

Question 14

Definition of lethal allele

Answer 14

A mutation of a gene that causes one genotype to die

Question 15

What is a Dihybrid cross

Answer 15

A dihybrid cross is how to determine the probability of genetic outcomes for TWO traits at the same time instead of ONE trait.

Question 16

What are single trait crosses called

Answer 16

MONOHYBRID crosses

Question 17

What are 2 key things to remember when drawing a dihybrid cross

Answer 17

Dominant alleles before recessive

Like alleles together.

Question 18

Example of a dihybrid cross:

Answer 18

Sperm: Ova:
TY TY
ty ty
Ty Ty
Yt Yt

      TY              ty             Ty             Yt TY     TTYY        TtYy         TTYy        TtYY ty       TtYy          ttyy          Ttyy          ttYy Ty      TTYy         Ttyy         TTyy        TtYy Yt       TtYY         ttYy          TtYy         ttYY

Offspring:
Phenotype: Tall yellow seeds: 9
Short yellow seeds: 3
Tall green seeds: 3
Short green seeds: 1
Phenotypic ratio: 9:3:3:1

Question 19

What is a linkage group

Answer 19

A linkage group is a group of genes whose loci are on the same chromosome & hence don’t independently assort.

Question 20

How will linked genes be inherited

Answer 20

Linked genes will tend to be inherited together & hence don’t follow normal Mendelian inheritance for a dihybrid cross. instead the phenotypic ratio will be more closely aligned to a monohybrid cross as the two genes are inherited as a single unit.

Question 21

When can linked genes become separated

Answer 21

Linked genes may become separated via recombination (due to crossing over during synapsis is meiosis 1)

Question 22

How are linked genes represented

Answer 22

Linked genes are represented as vertical pairs:
AB ——— ab

Question 23

What is a linked gene

Answer 23

Linked genes are genes located together on a chromosome and are therefore inherited together.

Question 24

How many gamete combinations are there for unlinked genes-dihybrid pattern

Answer 24

Unlinked genes-Dihybrid pattern= 4 possible gamete combinations

Question 25

What are gene loci said to be if they are on the same chromosome

Answer 25

Gene loci are said to be linked if on the same chromosome

Question 26

How many gamete combinations are there for linked genes-Monohybrid pattern

Answer 26

Linked genes-Monohybrid pattern= 2 possible gamete combinations

Question 27

What did Thomas Hunt Morgan do

Answer 27

He provided a key contribution to our current understanding of gene linkage by discovering non-mendelian ratios in fruit flies. His breeding experiments involving fruit flies clearly demonstrated that linked genes were NOT independently assorted.

Question 28

What did Thomas Hunt Morgan discover to do with sex linkage

Answer 28

When cross breeding red-eyed wild types with white-eyed mutants, he discovered a clear sex bias in phenotypic distribution.

All female offspring of a red eyed were red red eyed, whereas all male offspring of a white eyed female were also white eyed.

Question 29

How did Thomas Hunt Morgan describe this sex linkage distribution pattern

Answer 29

Morgan described this distribution as ‘sex limited’ inheritance and inferred it was caused by the gene for eye colour being located on a sex chromosome.

Question 30

Thomas Hunt Morgan found out (about gene linkage) that certain phenotypic combinations occurred in much lower frequencies than was to be expected. Based on this he made 2 key proposals. What are they?

Answer 30

1) The alleles for these traits were located on a shared chromosome (gene linkage) and hence did not independently assort.
2) Linked alleles could be uncoupled via recombination (crossing over) to create alternative phenotypic combinations, but these new phenotypes would occur at a much lower frequency.

Question 31

As well as Thomas Hunt Morgan’s 2 key proposals what else did he observe

Answer 31

He also observed that the amount of crossing over between linked genes differed depending on the combination of traits. This led to the idea that crossover frequency may be a product of the distance between two genes on a chromosome. Genes with a higher crossover frequency are further apart, whereas genes with a lower crossover frequency are closer together.

Question 32

What important thing did Thomas Hunt Morgan declare after completing his research

Answer 32

That genes located on the same chromosome (linked gens) will be inherited together, independent assortment does not apply.

Question 33

Why do linked genes not independently assort?

Answer 33

Because the genes are close together on the chromosome they are inherited together as a pair and therefore do not independently assort because they are linked.

Question 34

What happens to the phenotypic ratio (to do with linked genes)

Answer 34

In linked genes there will be a higher similarity between parent & offspring as a higher number of offspring will have the parent’s genotypes rather than the recombinant genotypes.

Question 35

What is the only way linked genes will become separated

Answer 35

The only way linked genes can be separated is through recombination which is where two homologous chromosomes align during meiosis and exchange a segment of genetic material.

Question 36

Describe the term multiple alleles

Answer 36

Multiple alleles are where there are 3 or more alleles for a gene.