Biology Reference
In-Depth Information
Maternal
chromosomes
Paternal
chromosomes
A
A
a
a
A
a
Gametes
A
a
F
1
generation
FIGURE 3-4.
Crossing a pure dominant parent and a pure recessive parent. Pea chromosomes in Mendel's first
cross, where AA and Aa produce smooth seeds and aa produce wrinkled seeds. The F
1
are all Aa
genotypically and are therefore smooth, which is the dominant phenotype.
A
a
A
a
F
1
A
a
A
a
or
or
Gametes
A
A
A
a
A
a
a
a
F
2
offspring occur in a 3:1 ratio of dominant (
AA, Aa, aA
) to recessive (
aa
)
FIGURE 3-5.
Diagram of the chromosomes in Mendel's cross of the F
1
generation. Observe the F
2
generation and
the chromosomal basis of the 3:1 ratio.
Punnett square, the paternal gametes are listed across the top and the
maternal gametes down the left side. The paternal and maternal gametes
are combined in an orderly fashion, with one maternal allele added to
one paternal allele in each cell of the Punnett square. As expected, we
see 1/4 AA,1/2Aa, and 1/4 aa or 3/4 dominant and 1/4 recessive, giving
the 3:1 ratio that Mendel observed.
