Page 313 - ABCTE Study Guide_Neat

• Dominant
When present, this allele will be expressed over others.
• Recessive
Traits that can be masked by a dominant allele
• Segregation
Each sperm or egg receives only one member of a pair of alleles.
• Homozygous
Both “units,” or alleles, are identical for a given trait.
• Heterozygous
A pair of alleles contains a dominant and a recessive gene.
• Independent Assortment
Pairs of alleles tend to be sorted into gametes independently of other pairs.

Today we know the “units” of inheritance Mendel described are genes carried on chromosomes. Human
chromosomes can carry thousands of genes, which are segments of DNA containing specific information
for assembling a sequence of amino acids during translation. We also know that clear dominance and
recessiveness do not describe all traits. Some can exhibit co-dominance or incomplete dominance, and
most traits are polygenic (coded by multiple genes).

We also know that independent assortment of traits occurs only when genes are carried on different
chromosomes. Independent assortment occurs because of the basically random alignment (and ability to
cross over) of chromosomes during metaphase one of meiosis.

Human Genetics

Humans have twenty-three pairs of chromosomes that must line up and separate. Chromosomal
disorders such as Down syndrome can occur when paired chromatids (in this case pair twenty-one) fail to
separate in meiosis and are then used in fertilization, leaving the offspring with an extra chromosome.

Traits carried on the twenty-third pair are called sex-linked; most are carried on the X chromosome and
known as X-linked. Unlike genes on other chromosomes, these do not pass with equal chance to male
and female offspring. Because males have only one X chromosome in pair twenty-three, they cannot pass
such traits down to male offspring.

This can be illustrated using the pedigree for color blindness shown below. Color blindness is an X-linked
trait expressed mainly in males.