Humans have small families but keep good medical records and sometimes
long genealogies by which inherited diseases may be studied.
- Humans are poor subjects for
genetic studies because their small family sizes makes genetic ratios
hard to determine. Also, humans cannot be mated just for experiments,
and long generation time limits the number of generations that can
be studied.
- Humans are good subjects for genetic studies in other ways:
thorough medical examinations and good medical records afford lots of
useful information, as do good genealogical records (family histories).
Human pedigrees show males as squares and females as circles.
Symbols are empty for "normal" individuals, filled in for affected
individuals. Horizontal lines indicate a marriage tie; lines leading
down from a marriage tie lead to offspring in the order of their birth.
Inheritance patterns for rare traits in which most people
are homozygous "normal":
- Dominant autosomal (non-sex-linked) traits:
- Assume that all individuals displaying the trait are heterozygous
- Homozygous individuals (very rare) can only come from parents
who both show the trait
- Recessive autosomal traits:
- Individuals showing the trait are always homozygous;
- their parents and children are heterozygous
(unless they show the trait also).
- If both parents show the trait, all their children will, too.
- If one parent shows the trait, and the other is homozygous normal,
then all children are heterozygous.
- If one parent shows the trait and one is heterozygous,
then half the children will show the trait and the others will
be heterozygous.
- The trait cannot appear for the first time unless heterozygotes marry, so these
traits show up more often if there are consanguineous
marriages (between genetically related persons).
- Dominant sex-linked traits:
- Sex-linked genes are always expressed in males.
- Females showing the trait are usually heterozygous.
- Recessive sex-linked traits:
- Sex-linked genes are always expressed in males.
- Affected males always outnumber affected females in family trees.
- Females never show the trait unless they are homozygous;
such females are rare and occur only if the father shows the trait.
- Except in such rare cases, mothers and daughters of affected
males are unaffected heterozygous individuals (carriers).
- Males with the trait do not have affected fathers or sons,
but they often have more remote male relatives (like grandfathers,
great-grandfathers, or grandsons) with the trait, and the connecting
individuals in the in-between generations are aways female.
Key examples of well-studied human traits:
- Biochemical deficiencies ("inborn errors of metabolism";
always recessive): Tay-Sachs disease, PhenylKetonUria (PKU),
albinism, hemophilia (sex-linked), fava bean disease (G6PD deficiency).
- Other single-gene traits:
- Huntington's chorea (autosomal dominant, often appearing around age 40-45)
- ability to taste PhenylThioCarbamide (PTC) (autosomal dominant)
- colorblindness (sex-linked recessive)
- sickle-cell anemia (autosomal recessive)
- A-B-O bloodgroups (autosomal, A and B dominant, O recessive)
- Chromosomal abnormalities:
- Down's syndrome (most often from an extra copy (trisomy)
of chromosome #21)
- Turner's syndrome (XO, a sterile female with only one X chromosome)
- Kleinfelter's syndrome (XXY, a thin, sterile male with an extra X chromosome)
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