4.3 TheoreticalGenetics

1
4.3 TheoreticalGenetics

• To be able to define the key terms of theoretical
  genetics
• To describe inheritance patterns using
  inheritance diagrams
• To be able to explain sex linkage

2
Definitions

• Genotype the alleles of an organism
• Phenotype The characteristics of an organism
• Dominant allele the allele that gets expressed
  in heterozygous condition
• Recessive allele the allele that has an effect
  on the phenotype when present in the homozygous
  state.
• Codominant alleles pairs of alleles that are
  both expressed when present
• Locus specific position of a gene in a
  chromosome
• Homozygous having two copies of the same allele.
• Heterozygous having two different alleles of a
  gene
• Carrier a person with one recessive faulty
  allele for a genetic disorder
• Test cross testing a suspected heterozygote by
  crossing with the homozygous recessive

3
Monohybrid Crosses

• Complete Dominance
• Incomplete Dominance
• Codominance

4
1 - Complete Dominance
P RR ? rr F1 Genotype Phenotype Type of
Genotype Rr red heterozygous F1 Cross Rr ?
Rr Punnett Square F2 Genotype Phenot
ype Type of Genotype RR red homozygous Rr
red heterozygous Rr red heterozygous rr
white homozygous
R
r
RR Rr
Rr rr
F2 Genotypic Ratio 1 RR 2 Rr 1 rr F2
Phenotypic Ratio 3 red 1 white
R
r
5
4 - Incomplete Dominance
P RR ? rr F1 Genotype Phenotype Type of
Genotype Rr pink heterozygous F1 Cross Rr
? Rr Punnett Square F2 Genotype Phen
otype Type of Genotype RR red homozygous R
r pink heterozygous Rr pink
heterozygous rr white homozygous
R
r
RR Rr
Rr rr
F2 Genotypic Ratio 1 RR 2 Rr 1 rr F2
Phenotypic Ratio 1 red 2 pink 1 white
R
r
6
7 - Codominance
P RR ? WW F1 Genotype Phenotype Type of
Genotype RW red white heterozygous F1
Cross RW ? RW Punnett Square F2 Gen
otype Phenotype Type of Genotype RR red homoz
ygous RW red white heterozygous RW red
white heterozygous WW white homozygous
R
W
RR RW
RW WW
F2 Genotypic Ratio 1 RR 2 RW 1 WW F2
Phenotypic Ratio 1 red 2 red white 1 white
R
W
7
ABO Blood Typing
8
ABO Blood Typing
9
ABO Blood Typing
10
ABO Blood Typing
RECIPIENT
A B AB O
A Y Y
B Y Y
AB Y
O Y Y Y Y
D O N O R
11
15 - Blood Typing
P IAi ? IBi Punnett Square F1 Ge
notype Phenotype IAIB AB IAi A IBi
B ii O
IB
i
IAIB IAi
IBi ii
IA
i
12
Additional Crosses

• Sex Determination
• Sex Linkage
• ABO Blood Typing

13
12 - Sex Linkage- Haemophilia
P XHY ? XHXh Punnett Square F1 G
enotype Phenotype Sex Type of Genotype XHXH nor
mal F homozygous XHXh carrier F heterozygous
XHY normal M hemizygous XhY hemophilia M hemiz
ygous
XH
Xh
XHXH XHXh
XHY XhY
XH
Y
14
11 - Sex Determination
P XY ? XX
X
X
XX XX
XY XY
X
Ratio 1 XX 1 XY
Y
15
Dihybrid Crosses

• Complete Dominance

16
8 - Complete Dominance
P yyrr ? YYRR F1 Genotype Phenotype Type of
Genotype YyRr yellow/round heterozygous F1
Cross YyRr ? YyRr Punnett Square F2
Genotype Phenotype 9 Y_R_ yellow/round 3
Y_rr yellow/wrinkled 3 yyR_ green/round 1
yyrr green/wrinkled
YR
Yr
yR
yr
YYRR YYRr YyRR YyRr
YYRr YYrr YyRr Yyrr
YyRR YyRr yyRR yyRr
YyRr Yyrr yyRr yyrr
YR
Yr
yR
yr
17
Pedigrees
18
Pedigrees
Normal
Affected
Carrier
19
Pedigrees
Example You are a normal female (homozygous).
Both of your parents are normal. Your younger
brother is normal and your older brother has
hemophilia.
20
18 - Pedigrees
You!