Inheritance

Question 1

Inheritance

Answer 1

The passing on of characteristics from one generation to the next.
Inherited characteristics (traits) of an individual are determined by genes which are passed from one generation to the next via sex cells.

Question 2

Alleles

Answer 2

The alternative variations (or forms) of the gene are called alleles (e.g. white and purple petal colour).

Question 3

Dominant alleles

Answer 3

Hide (mask) the expression of recessive alleles. In this form of inheritance alleles are symbolized by using the same letters of the alphabet: dominant alleles by using capital letters (eg. B) and recessive alleles by using lower case (eg. b).

Question 4

Recessive alleles

Answer 4

Hidden (masked) in the presence of the dominant trait and can only be expressed if the allele for the dominant trait is not present.

Question 5

Genotype

Answer 5

The combination of alleles that an organism has. (It is the symbolic representation of the genes (alleles) possessed for a particular trait). The genotype of an individual for one trait is depicted using allele symbols.

Question 6

Homozygous

Answer 6

If both copies of a gene are the same (e.g. BB or bb), that individual is described as being homozygous

Question 7

Heterozygous

Answer 7

If both copies of the gene are different (e.g. Bb) then they are described as heterozygous for that gene.

Question 8

Phenotype

Answer 8

The phenotype is the visible effect of the allele combinations possessed (eg. brown eye colour). The phenotype can also be influenced by the environment. Factors in the environment such as availability of nutrients and water can sometimes influence how genes are expressed.

Question 9

Rules for drawing and interpreting pedigrees

Answer 9

The sex of an individual is indicated by shape. Females are always drawn as circles and males are always squares.

To show the marriage or breeding relationship between individuals a line is drawn, connecting the male and female as a couple.

Children (offspring) are indicated using a line, which is drawn from the breeding couple/marriage line. The eldest child is always drawn first (to the left) and youngest last (to the right).

Carriers’ of a particular trait may be indicated by a dot in the centre of the respective shape.

Individuals that have the particular characteristic of interest are shaded in (whether dominant or recessive).

The rows of the pedigree represent different generations. The top row of the pedigree represents the first generation. When they reproduce they give rise to the second row (second generation) and so on.

Question 10

Monohybrid crosses

Answer 10

Indicate the inheritance of one gene (trait) at a time (Genotypes have two letters eg. Bb)

Question 11

Dihybrid crosses

Answer 11

Indicate the inheritance of two genes at a time (Genotypes have four letters eg. BbTt)

Question 12

Incomplete (Partial or Blended) Inheritance

Answer 12

Both alleles are partially expressed.

Heterozygous condition is blended

Question 13

Codominance Inheritance

Answer 13

Both alleles are fully expressed

Heterozygous condition is a new phenotype

Use different symbols of alphabet for genotypes as superscripts to a gene symbol eg. CR CW

Question 14

Co-dominance Inheritance

Answer 14

An inheritance pattern where different alleles are equally expressed (one does not mask the other). This results in the heterozygote form showing characteristics of both parents (a new phenotype). In ABO blood grouping, both the ‘A’ and ‘B’ alleles are co-dominant to each other.

Question 15

Multiple Allele Inheritance

Answer 15

A choice of more than two different alleles can be inherited for a gene.

Question 16

Sex-linked Inheritance

Answer 16

The genes for some traits are located on the sex chromosomes

X-linked inheritance occurs when the gene is located on the X sex chromosome (The X chromosome contains 2000 genes)

Y-linked inheritance occurs when the gene is located on the Y sex chromosome (The Y chromosome contains 65 genes).

phenotypes specify the person’s sex (e.g. colour blind male).

genotypes include the sex chromosome symbols (a female is denoted by XX and a male is denoted as XY).

genotypes include specific allele symbols as superscripts next to the X sex chromosome. (A female could be written as XHXH (homozygous) or XHXh (heterozygous)).

genotypes do not include superscripts written on the Y chromosome. (ie. there are no genes on the Y chromosome to mask those on the X chromosome)

Question 17

Polygenic Inheritance

Answer 17

When one characteristic is controlled by two or more genes
raits are quantitative (features vary over a range rather than as an either/or manner.) Phenotype depends on a combination of genes with additive effects.

two or more genes contribute to the genotype and these genes often exhibit incomplete dominance.

many phenotypes exist (bell shaped curve).

in combinantion with environmental effects often results in continuous variation of phenotypes.

human examples include: skin colour, eye colour, height, weight

Question 18

Phenotypic variation

Answer 18

Discontinuous (discrete) variation - where phenotypes are distinct (as often seen with monogenic inheritance eg. pea seed shape, blood grouping)

OR

Continuous variation - is often where there is a range of phenotypes which when shown on a graph display a bell shaped curve (as often seen with polygenic inheritance eg. skin colour, wheat grain colour)