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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
XX
XX
X
X
Y
XY
XY
There is a random chance of which sperm will
fertilise the egg. There is therefore a 50-50
chance that the offspring will be male or female.
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
The diagram expressed to the left is a monohybrid cross
showing the likelihood of having a child with blue eyes
(bb) or having a child with the recessive blue eye allele
but not actually blue eyes (Bb). The recessive alleles are
represented by lower-case letter and the dominant
alleles are represented by upper-case letters.
Variation and Inheritance – Revision Pack (B1)
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.
Variation and Inheritance – Revision Pack (B1)
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