Variation and Inheritance Revision Pack (B1)

Variation and Inheritance – Revision Pack (B1)

Inherited characteristics:

Some human characteristics, such as facial features and eye colour, can be inherited. They can be either dominant or recessive.

Alleles are different versions of the same gene.

Many people believe that intelligence, sporting ability and health are inherited factors, while others believe that the environment in which someone lives influences these characteristics. This debate is known as “nature vs. nurture”.

Dominant and recessive characteristics are dependent on dominant and recessive alleles. Dominant alleles will be noticed if they are present, but recessive alleles are only noticed in the absence of the dominant allele.

NOTE – a gene can have two different alleles.

Chromosomes:

Most body cells will have the same number of chromosomes. The number will depend on the species of organism. Human cells have 23 pairs (46 chromosomes in total).

Sex chromosomes determine sex in mammals. Females have identical sex chromosomes (XX). Males have different sex chromosomes (XY).

A sperm will carry EITHER an X OR a Y chromosome. All eggs will carry an X chromosome.

X / Y
X / XX / XY
X / XX / XY

Genetic Variation:

Genetic variation is caused by:

1) Mutations, which are random changes to genes or chromosomes

2) The rearrangement of genes during the formation of gametes (sex cells)

3) Fertilisation, which results in a zygote (fertilised ovum) with a mix of alleles from the mother and the father

A Monohybrid Cross:

b / b
B / Bb / Bb
b / bb / bb

Homozygous means having the same alleles (e.g. bb), while heterozygous means having different alleles (e.g. Bb).

A genotype is the person’s genetic makeup, for example if they had blue eyes, their genotype would be bb.

A phonotype is how this is actually seen as a characteristic, or which alleles are actually expressed, for example, using the above example; the person’s phonotype is blue eyes.

Inherited disorders:

Inherited disorders are caused by faulty genes.

There are many personal and ethical issues when it comes to finding out that a parent carries a faulty gene:

- For example, deciding whether or not to have a genetic test – a positive result for a faulty gene could alter lifestyle and career

- For example, deciding whether you want to marry and whether you should take the risk and have a child

Inherited disorders are caused by faulty alleles, which are almost always recessive.

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