Genetics & Heredity
Meiosis
• Resulting cells have a different genetic
configuration than the original cell.
• End product is 4 haploid gametes
• 2 divisions occur, not 1
• Crossing over may occur
• 1 replication with 2 divisions
Independent assortment
• Genes on different chromosomes behave
independently during meiosis.
• Separate chromosomes will separate into
gametes independently of each other.
Meiosis
parents
gametes
Male gametes
Female gametes
Punnet Squares
Tt x Tt= genotype
T t T t= gametes
Punnet Squares
Tt x Tt= genotype
T t x T t= gametes
Male gametes
t
T
Punnet Squares-monohybrid cross
Female Gametes
Tt x Tt= genotype
T t x T t= gametes
T
t
Male gametes
T
t
Punnet Squares-monohybrid cross
Female Gametes
Tt x Tt= genotype
T t x T t= gametes
T
t
Male gametes
T
TT
t
Punnet Squares-monohybrid cross
Female Gametes
Tt x Tt= genotype
T t x T t= gametes
T
t
Male gametes
T
t
TT
Tt
Punnet Squares-monohybrid cross
Female Gametes
Tt x Tt= genotype
T t x T t= gametes
Male gametes
t
T
T
TT
t
tT
Tt
Punnet Squares-monohybrid cross
Female Gametes
Tt x Tt= genotype
T t x T t= gametes
Male gametes
T
t
T
TT
Tt
t
tT
tt
Punnet Squares-monohybrid cross
Female Gametes
Tt x Tt= genotype
T t x T t= gametes
Male gametes
T
t
T
TT
Tt
t
tT
tt
Mendelian Ratios:
Genotypic: 1TT: 2 Tt: 1tt
Phenotypic: 3 dominant: 1
recessive
Chromosomes and Sex Determination
• Human sperm contains 23 chromosomes,
22 of which are autosomes and 1 which is
the sex chromosome. Half the sperm
have an “X” sex chromosome, the other
1/2 has a “Y” sex chromosome.
• The ovum also contains 22 autosomes
and 1 sex chromosome; it’s sex
chromosome has been designated “X”.
• XX zygote= female : XY zygote= male
Gamete combination possibilities
• In a situation with Green (yy) and Round
(RR) versus yellow (YY) and wrinkled
(rr)
• gametes would be yy X RR or 4 yR
– 4 Yr Heterozygotes for these traits would be
– Rr X Yy and the four gametes would be:
• YR
Yr
yR
yr
Dihybrid Cross
Genotypes: RrYy
x
Gametes: What are the gametes
RrYy
Dihybrid Cross
Genotypes: RrYy
Gametes: RY Ry rY ry
x
RrYy
RY Ry rY ry
Dihybrid Cross
Genotypes: RrYy
Gametes: RY Ry rY ry
RY
RY
Ry
rY
ry
x
Ry
RrYy
RY Ry rY ry
rY
ry
Dihybrid Cross
Genotypes: RrYy
Gametes: RY Ry rY ry
RY
Ry
rY
ry
x
RY
Ry
RRYY
RRYy
RrYy
RY Ry rY ry
rY
ry
Dihybrid Cross
Genotypes: RrYy
Gametes: RY Ry rY ry
x
RrYy
RY Ry rY ry
RY
Ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
rY
ry
Dihybrid Cross
Genotypes: RrYy
x
Gametes: RY Ry rY ry
Ry
RY
RrYy
RY Ry rY ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
Mendelian Ratios: R-Y- 9/16
R-yy- 3/16
rrY- 3/16
rryy- 1/16