Topic 7—A: Genetics- 2. Genetic diagrams— simple monohybrid cross

Question 1

What do genetic diagrams show?

Answer 1

How alleles could be passed on to the next generation

Question 2

What are diploid organisms?

Answer 2

• they have two alleles for each gene

Question 3

Definition of haploid

Answer 3

-when a cell contains one copy of each chromosome
- gametes (sex cells) contain only one allele. For each gene

Question 4

What happens when the haploid gametes from two parents fuse together?

Answer 4

• the alleles they contain form the genotype of the diploid offspring that is produced

Question 5

What can genetic diagrams be used to predict?

Answer 5

• the genotypes and phenotypes of the offspring produced if two parents are crossed (bred)

Question 6

Definition of monohybrid inheritance

Answer 6

• the inheritance of a single characteristic controlled by a single gene

Question 7

What do monohybrid crosses show?

Answer 7

• the likelihood of the different alleles of that gene (and so different versions of the characteristic) being inherited by offspring of certain parents

Question 8

Example of monohybrid crosses

Answer 8

• allele for normal wings is dominant so its shown by a capital letter N
• Any flies that have even one N allele will have normal wings
• allele for vestigial (little) wings is recessive so its show by the letter n
• Only flies that have two n alleles will have vestigial wings

N- normal wings allele n- vestigial wings allele

Parents genotypes NN nn
Gametes. Nn Nn. Nn. Nn
They are all heterozygous as one allele is inherited from each parent
F1 parents genotypes- Nn Nn
Gametes alleles- N n N n
NN Nn Nn nn- possible genotypes of F2
Normal, normal, normal, vestigial (Phenotypes)

Question 9

What is the phenotypic ratio?

Answer 9

The ratio of different phenotypes in the offspring
Genetic diagrams allows you to predict the phenotypic ratios in F1 and F2 offspring

Question 10

What is the usual phenotypic ratio whenever you do a monohybrid cross with two heterozygous parents?

Answer 10

• you get a 3:1 ratio of dominant:recessive characteristics in the offspring

Question 11

What is a punnet square?

Answer 11

• a punnet square is just another way of showing a genetic diagram

Question 12

Steps of a punnet square

Answer 12

Step 1- work out the alleles that gametes would have
Step 2- cross the parents gametes to show the possible genotypes of the F1 generation
Step 3- cross the gametes of the F1 generation to show the possible genotypes of the F2 generation

Question 13

What do alleles occasionally show?

Answer 13

Codominance

Question 14

What does codominance mean?

Answer 14

• both alleles are expressed in the phenotype and neither one is recessive

Question 15

What is an example of codominance in humans?

Answer 15

The allele for sickle cell anaemia

Question 16

What is sickle cell anaemia?

Answer 16

A genetic disorder caused by a mutation in the haemoglobin gene
- it causes red blood cells to be sickle shaped

Question 17

Example of monohybrid inheritance of codominant alleles

Answer 17

• people who are homozygous for normal haemoglobin (HN HN) don’t have the disease
• People who are homozygous for sickle haemoglobin (HS HS) have sickle cell anaemia- all their blood cells are sickle shaped
• People who are heterozygous (HN HS) have an in between phenotype called the sickle cell trait- they have some normal haemoglobin and some sickle haemoglobin
• The two alleles are codominant because they’re both expressed in the phenotype

Possible offspring from crossing two parents with sickle-cell trait
Parents genotypes- HNHSXHNHS
Gametes alleles- HN HS HN HS
Possible genotypes of offspring- HNHN, HNHS, HNHS, HSHS
Phenotypes of offspring— unaffected, sickle-cell trait, sickle-cell trait, sickle-cell anaemia
1:2:1 phenotypic ratio

Question 18

Whenever you do a monohybrid cross with two heterozygous parents involving codominant alleles what would you expect to see?

Answer 18

A 1:2:1 ratio in the offspring