Genes And Inheritance

Question 1

What is meant by homozygous and heterozygous?

Answer 1

Homozygous- where both alleles of a gene are identical
Heterozygous- where both alleles of a gene are different

Question 2

What are dominant and recessive alleles?

Answer 2

Dominant alleles are always expressed in the phenotype if present in the genotype
Recessive alleles must be homozygous to be expressed in the phenotype

Question 3

What features must an organism have in order for inheritance to be investigated ?

Answer 3

Small size
Short life span
Many offspring
Categoric characteristics
Be able to reproduce sexually

Question 4

What are monohybrid crosses?

Answer 4

Inheritance of one gene that codes for one characteristic

Question 5

What are the headings in a genetic diagram?

Answer 5

Parent phenotype
Parent genotype
Gametes
F1 genotype
F1 phenotype

Question 6

How can you determine an unknown genotype that displays a dominant trait?

Answer 6

Cross it with a homozy recessive organism. If the offspring all have the dominant trait the geneotype is dominant, of some show the recessive train it was heterozygous

Question 7

Why might using crossing to determine genotypes not always be accurate?

Answer 7

Due to random fertilisation of gametes- for example heterozygous parents may still only produce dominant offspring

Question 8

What is codominance?

Answer 8

Where alleles in a heterozygous organism are both expressed in the phenotype because they are both dominant. This therefore produces a third phenotype that is possible

Question 9

What happens when genes have more than two alleles?

Answer 9

Theyre combinations of alleles can be more varied

Question 10

What is sex determination?

Answer 10

Where an organisms sex is determined by the combination of chromosomes in an organism rather than genes.

Question 11

What are autosomes?

Answer 11

Non- sex determining chromosomes

Question 12

What is sex linkage?

Answer 12

Where genes that are carried on the sex chromosomes are inherited differently between males and females

Question 13

Why does sex linkage occur?

Answer 13

Y chromosomes are shorter than X chromosomes and so can carry less gene content than X chromosomes.
In Males, many genes on the X chromosome do not have an equivalent on the Y chromosome, and so only one recessive gene on the X chromosome needs to be present to be expressed in the males phenotype. Yet two need to be present in females.

Question 14

Give two examples of sex linkage diseases

Answer 14

Haemophilia (blood clotting) and colour blindness

Question 15

Using pedigree diagrams, what evidence may show sex linkage?

Answer 15

If there are more affected males than females, or no affected females.

Question 16

Using bedigree diagrams, what evidence can be against sex linkage?

Answer 16

When females with a recessive phenotype have a son or father with a dominant phenotype

Question 17

What are dihybrid crosses?

Answer 17

Where two different genes with two different characteristics are inherited, as organisms have four alleles in their genotype, which produces gametes with two alleles each.

Question 18

What is the dihybrid ratio?

Answer 18

9:3:3:1 and is seen when two organisms that are heterozygous for both genes are crossed

Question 19

What is epistasis?

Answer 19

Occurs in dihybrid crossing when there is an interaction between the two genes, as one gene suppresses the other. This is called one gene being epistatic over the other

Question 20

What is autosomal linkage?

Answer 20

Where genes on the same chromosome are linked, as they stay together during independent assortmant of chromosomes. Alleles are passed pn tegether and fewer types of gametes are formed

Question 21

During dihybrid crosses, why might the dihybrid ratio not be seen?

Answer 21

Due ro epistasis, autosomal linkage or sometimes crossing over