Unit A Reproduction

allele for
straight
hairline
homologous
chromosomes
gene for
hairline
allele for
widow’s
peak
Figure 2 The different forms of a gene
are called alleles. In any one individual,
there are only two alleles for every
gene—one allele on each homologous
chromosome.
chromosome has a corresponding gene, these genes may be slightly different
from one another. The different forms of the same gene are called alleles
(Figure 2). In humans, for example, homologous chromosomes may contain
two different alleles for the gene that controls the shape of the hairline: one
chromosome has the allele for a widow’s peak and the other chromosome
has the allele for a straight hairline (Figure 3). The allele for a widow’s peak
is dominant. A dominant allele will express its trait (the trait will show up in
a person’s appearance) if it is present. The allele for a straight hairline is
recessive. A recessive allele will only be expressed if both chromosomes
contain the recessive allele. If an individual has one of each allele, the
dominant trait is expressed.
(a)
(b)
Figure 3 Humans can have either a widow’s peak (a) or a straight hairline (b).
Figure 4 These horses are both roans.
They have two colours of hairs in their
coats. One has a mix of black hairs and
white hairs. The other has a mix of
brown hairs and white hairs.
In carnations, when a plant has one allele for red flowers and one allele for
white flowers, the result is pink flowers. This is incomplete dominance: two
different alleles produce a mixture of the traits. Codominance occurs when
both traits are expressed in one individual. An animal born with an allele for
white hair and an allele for red hair will have a roan-coloured coat, with a
mix of white hairs and red hairs (Figure 4). In both cases, each parent has
contributed only one of its two alleles in its sperm cell or egg cell.
Some characteristics are controlled by more than one gene or there are more
than two alleles of the gene. In these cases, the combination of the two alleles
from both parents can produce an entirely different trait in the offspring. Eye
colour in humans is one example. The amount of pigment and the pattern of
pigment in the eye are controlled by several genes. The incredible process of
sexual reproduction produces the different combinations that account for the
vast diversity among living things.
Comparing Sexual and Asexual Reproduction
Sexual reproduction is much more complicated than asexual reproduction.
Does this complexity mean that sexual reproduction is better than asexual
reproduction? Both methods of reproduction have existed for millions of
years, and both methods have advantages and disadvantages.
80 Unit A Reproduction
NEL