Site author Richard Steane
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The BioTopics website gives access to interactive resource material, developed to support the
learning and teaching of Biology at a variety of levels.
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GENETIC PEDIGREES
("Family trees")
In these diagrams, people are represented by symbols, usually circles for female and squares for male, and the bottom line represents the children of the couple above.
For simplicity, 4 offspring are shown in these examples. However, in practice the number, proportion and order of birth are likely to vary.
Obviously, the same technique of family trees can be used to show the results of animal breeding.
It is customary to use dark symbols to indicate someone affected with a genetic condition, and unfilled symbols for those who are unaffected. In the explanations that follow, an intermediate grey colour is used to assist in understanding the effect of a heterozygote carrier.
Dominant allele, e.g. Huntington's disease (Huntington's chorea)
Genetic explanation
Since the condition is shown in some of the first generation offspring but not in some others, this is not a simple cross between 2 different homozygotes. One parent must be heterozygous, and the allele for the condition must be dominant to the allele for absence of the condition. The dominant allele is denoted by an upper case letter e.g. H as distinct from the lower case version of the same letter h for the normal allele (recessive, in this case).
Note also that in this case the appearance of the condition is independent of the sex of the individual.
Genetic diagram
Parents
phenotype |
affected |
x |
unaffected |
Parents
genotype |
Hh |
|
hh |
Gametes |
H or h |
h only |
Genotypes of offspring |
Hh |
& |
hh |
Phenotypes of offspring |
1 affected
(50%) |
: |
1 unaffected
(50%) |
Recessive allele, e.g Cystic fibrosis
Genetic explanation
Since the condition is not shown in any of the offspring in the first generation but it reappears in the second generation, it must be caused by a recessive allele. The recessive allele is denoted by a lower case letter e.g.
f as distinct from the upper case version of the same letter F for the normal allele (dominant, in this case).
Once again, in this case the appearance of the condition is independent of the sex of the individual.
Genetic diagrams
For the first section (parents giving rise to the first generation):
Parents
phenotype |
affected |
x |
unaffected |
Parents
genotype |
FF |
|
ff |
Gametes |
F only |
f only |
Genotype of offspring |
|
All
Ff |
|
Phenotype of offspring |
|
unaffected
- carriers |
|
For the second section ( two carriers giving rise to the second generation):
First generation:
phenotype |
unaffected
- carrier |
x |
unaffected
- carrier |
First generation:
genotype |
Ff |
|
Ff |
Gametes
|
F or |
f |
|
F or |
f |
Genotypes of second generation offspring
|
FF |
Ff |
Ff |
ff |
Phenotypes of second generation offspring |
3 unaffected
- 2/3 carriers
- 1/3 completely
unaffected |
: 1 affected |
Sex-linked recessive allele, e.g haemophilia
(also applies to red-green colour blindness)
Example 1
Example 2
Genetic explanation
Since there are different results depending on the sex of the parents in these 2 examples, this allele must be sex-linked, and carried on the X chromosome. Females (genotype XX) have 2 copies of this chromosome so dominance can occur, resulting in symptomless carriers. Males (genotype XY) have only one X chromosome so they are either affected or unaffected. There are no carrier males.
The recessive allele on the X chromosome is denoted by a superscript lower case letter e.g. Xh as distinct from the upper case version of the same letter XH for the normal allele (dominant, in this case). In males, a Y chromosome, not carrying an equivalent gene, is also present.
Example 1
Parents
phenotype |
unaffected
female |
x |
affected
male |
Parents
genotype |
XHXH
|
|
XhY |
Gametes
|
XH
only |
|
Xh
or |
Y |
Genotypes of offspring
|
XHXh |
|
XHY |
Phenotypes of offspring |
All females unaffected
- but carriers |
All males
unaffected |
Example 2
Parents
phenotype |
affected
female |
x |
unaffected
male |
Parents
genotype |
XhXh
|
|
XHY |
Gametes
|
Xh
only |
|
XH
or |
Y |
Genotypes of offspring
|
XHXh |
|
XhY |
Phenotypes of offspring |
All females unaffected
- but carriers |
All males
affected |
Example 3
Example 3
Parents
phenotype |
carrier
female |
x |
unaffected
male |
Parents
genotype |
XHXh
|
|
XHY |
Gametes
|
XH
or |
Xh |
|
XH
or |
Y |
Genotypes of offspring
|
XHXH |
XHXh |
|
XHY |
XhY |
Phenotypes of offspring |
ALL females
unaffected - but:
1/2 carriers
1/2 free of allele |
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1/2 males unaffected
1/2 males affected |