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GENETIC PEDIGREES
("Family trees")



Basic
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)



Dominant allele family tree

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 V 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


Family tree for recessive allele

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 V 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
V
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

Sexl1
Example 2
Sexl2

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
V
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
V
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


Sexl3


Example 3

Parents
phenotype
carrier
female
x unaffected
male

Parents
genotype

XHXh

V
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

1/2 males unaffected
1/2 males affected



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