Inheritance

Question 1

what is a gene?

Answer 1

A sequence of DNA that codes for a polypeptide

Question 2

Define allele

Answer 2

different sequences
A single gene may have one, two or many alleles.
Alleles always occupy the same locus because they are different versions of the same gene

Question 3

diploid individuals

Answer 3

has one of each chromosome from each parent and, therefore, one copy of each gene from each parent

Question 4

Define homozygous

Answer 4

If the alleles of a particular gene are the same from both parents, the individual is homozygous for that gene

Question 5

Define heterozygous

Answer 5

If the alleles of a particular gene are different from both parents, the individual is heterozygous for that gene

Question 6

Define genotype

Answer 6

All the alleles they contain

Question 7

Define phenotype

Answer 7

their expression of the genotype in a specific environment

Question 8

Define dominant

Answer 8

The allele that is always expressed when it is present

Question 9

Define recessive

Answer 9

The allele that is only expressed when it is homozygous

Question 10

Define monohybrid inheritance

Answer 10

The inheritance of a single gene

Question 11

Peas were a useful choice for experiments because

Answer 11

• They are easy to grow
• Their flowers can self-fertilise and cross-fertilise
• They make flowers and fruit in the same year
• They make a large number of seeds from each cross. This means that when the phenotypes of the next generation are counted, their numbers make them statistically meaningful

Question 12

Mendel chose characteristics that are:

Answer 12

• Controlled by single genes
• Controlled by genes on different chromosomes
• Clear cut and easy to tell apart

Question 13

Describe continuous variation

Answer 13

They have a range of values and are controlled by a number of genes, for instance height, in humans

Question 14

A diagram of a genetic cross show

Answer 14

• The generations, i.e. parents, first generation (F1), second generation (F2) and, sometimes further generations
• The genotypes of parents and offspring
• The alleles present in the gametes
• The symbols for the alleles are defined

Question 15

What is the F1 (first filial) generation

Answer 15

It is the offspring of the parents of the cross.
F2 stands for the second filial generation. It is the offspring of a self- fertilised F1 plant or a cross between members of the F1 generation. It is also the grandchildren of the original parents

Question 16

Define fillial

Answer 16

Relating to a son or daughter

Question 17

Instructions for writing a diagram for a genetic cross

Answer 17

1. Choose suitable symbols for the alleles
   - Use a single letter for each characteristic, for example, the first letter of one of the contrasting features
   - If possible, use a letter in which the upper and lower cases differ in shape and size
   - Use the upper case for the dominant characteristic and the lower case letter the recessive
2. Write the parents’ genotypes with the appropriate pairs of letters. Label ‘Genotype of parents, and state their phenotypes
3. Show the gametes produced by each parent. Circle and label them ‘Gametes’
4. Use a matrix, called a punnet square, to show the results of the possible combinations that result from random crossing of all the gametes. Label ‘Genotype of F1’
5. Show the phenotype of each F1 genotype in the Punnet square
6. Indicate the ratio of the phenotypes

Question 18

How can you tell if a dominant characteristic is determined by one of two dominant alleles?

Answer 18

By carrying out a test cross

Question 19

How do you do test crosses work?

Answer 19

Cross the heterozygous alleles with the unknown

E.g. If the tall plant were TT, all of the F1 would be tall, but if it were Tt, 50% would be tall and 50% would be dwarf

Question 20

What is co-dominance?

Answer 20

Both alleles in a heterozygote are expressed individually

As a result, the heterozygote has a combination of both of the homozygotes’ characteristics

Question 21

What is incomplete dominance?

Answer 21

For some genes, the phenotype of the heterozygote is intermediate between the two parental phenotypes, rather than their both being expressed

Question 22

1st law of inheritance- the law of segregation

Answer 22

states that the characteristics of an organism are determined by factors (alleles) which occur in pairs
Only one pair is present in each gamete

Question 23

What are unlinked genes?

Answer 23

Genes that behave independently in relation to each other

Question 24

Define dihybrid inheritance

Answer 24

The simultaneous inheritance of two unlinked genes, i.e. genes on different cromosomes

Question 25

How would you carry out a dihybrid test cross?

Answer 25

Testing a genotype by crossing it with an individual which is homozygous recessive for both genes The ratios of phenotypes in the progeny indicate the genotype of the parent

Question 26

Why are some genes described as linked?

Answer 26

The alleles of two genes that are on the same chromosome cannot segregate independently, i.e. they cannot move to opposite poles of the cell at meiosis This is because they are on the same physical structure- the chromosome- so they must move together

Question 27

The further apart two genes are on a chromosome | what does this lead to?

Answer 27

the more opportunity there is for crossover to occur between them. This leads to more recombinant gametes and therefore more offspring with recombinant phenotypes

Question 28

Recognising linkage

Answer 28

If the numbers of progeny with different combinations of characteristics do not correspond with Mendelian ratios, it is likely that the genes are linked

Question 29

What is a common explanation for rejecting the null hypotheses?

Answer 29

If the numbers of progeny with different combinations of characteristics do not correspond with Mendelian ratios, it is likely that genes are linked

Question 30

What is the expected ratio of phenotypes in a dihybrid cross?

Answer 30

9:3:3:1

Question 31

What is the null hypothesis?

Answer 31

This states that there is no difference between the observed and expected frequencies

Question 32

The reason that this is a null hypothesis is that Mendelian inheritance is based on there being no difference in:

Answer 32

- The number of the different types of gamete - The probability of each gamete type fusing with another type of gamete - The viability of the embryos, whatever their genotype - And that genes are not linked