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Diploid: pair of homologous chromosomes, two genes (BB)
Haploid: single copy of a gene (B)
Genotype: The pair of genes (BB, Bb, bb)
homozygous
dominant
heterozygous
homozygous
recessive
Phenotype: The expression of the genotype (colour, height, disease, etc). "What it looks like"
Dominant: the phenotype that shows over the recessive gene
b
B
phenotype
phenotype
Smooth seeds in peas (S) is dominant over wrinkled seeds (s) in peas. Draw a Punnett square and show the possible genotypes if a heterozygous smooth seeded plant (Ss) was crossed with a wrinkle seeded plant (ss). (4)
genotype
genotype
represents possible gamete
gene combinations (haploid)
like a multiplication chart
12. Say that black fur in dogs (B) is dominant over brown fur (b), and floppy ears (F) are dominant over upright ears (f). What are the proportions of the phenotypes if you cross two dogs that are heterozygous for both traits? (4)
Bb and Ff, so we put them together as a "dihybrid"
BbFf x BbFf, now we have 4 possible gamete gene combinations
so it's a bigger punnet square
We do the math FOIL to
make sure we get all the
combinations. Remember
you want one gene of each
trait
first
last
er
er
inn
BF
out
BbFf
Bf
bF
bf
9:3:3:1
B_F_
B_ff
bbF_
black floppy black upright brown upright
bbff
brown floppy
What would be the proportions of possible genotypes and phenotypes if you were to cross a dog that was heterozygous for the two traits of fur colour and ears with a dog that was homozygous recessive for both traits? (8)
bbff
BbFf
These columns
are all the
same, because
the gene
combinations
(bf) are all
the same. To
save time, you
don't have to
do all the
rows.
4:4:4:4 is the same as 1:1:1:1
so always write it in lowest terms
BbFf
Bbff
bbFf
black floppy
black upright
brown floppy
bbff
brown upright