biology 12
DESCRIPTION
Biology 12. Unit 3: Heredity Mendelian Genetics. Genes and Heredity. Mendel’s Experiments Alleles Punnett Square Monohybrid & Dihybrid Crosses. Mendelian Genetics. Traits are controlled by genes Law of Dominance: dominant and recessive Law of Segregation: genes separate during meiosis - PowerPoint PPT PresentationTRANSCRIPT
Genes and Heredity
• Mendel’s Experiments
• Alleles
• Punnett Square
• Monohybrid & Dihybrid Crosses
Mendelian Genetics
• Traits are controlled by genes
• Law of Dominance: dominant and recessive
• Law of Segregation: genes separate during meiosis
• Law of Independent Assortment: genes for one trait will separate independently of how other traits separate
More Mendelian
• Mendel worked with pea plants• True breeding tall plants are homozygous for
the tall (T) allele• True breeding dwarf plants are homozygous
for the dwarf (t) allele
• When the parental generation (P1) was made up of true breeding tall and true breeding dwarf plants all offspring were tall
P1 phenotype: Tall X Dwarf
genotype: TT X tt
gametes: T t
F1 phenotype: Tall X Tall
genotype: Tt X Tt
gametes: T t T t
F2 phenotype: 3 Tall : 1 dwarf
genotype: 1 TT : 2 Tt : 1 tt
Punnett Square
• This F1 generation of tall plants were all heterozygous: one tall allele and one dwarf allele
• When F1 individuals were crossed Mendel found a 3:1 ratio of tall to short plants
T
t
T
Tt
Tt
Tt
Tt
t
T
T
t
Tt
Tt
tt
TT
t
Possible Crosses
D
D
d
Dd
Dd
dd
DD
d
E
e
e
Ee
ee
ee
Ee
e
C
c
C
Cc
Cc
Cc
Cc
c
A
A
A
AA
AA
AA
AA
A
B
B
B
Bb
BB
Bb
BB
b
f
f
f
ff
ff
ff
ff
f
Test Cross
• When an individual showing the dominant phenotype is crossed with a homozygous recessive individual to determine the genotype
t
T
t
?t
Tt
?t
Tt
?
If dwarf individuals are observed in the offspring, then the unknown genotype is heterozygous
Dihybrid Crosses
• Mendel looked at two traits at a time to determine the Law of Independent assortment
• True breeding Yellow Round Seed cross with true breeding green wrinkled seed
Let Y be yellow, y is greenLet R be round, r is wrinkled
P1: genotype YYRR X yyrr gametes YR yrF1: genotype YyRr X YyRr gametes YR Yr yR yr
F2:YR
YR Yr yR yr
yr
yR
Yr
Dihybrid Crosses
YR
YR Yr yR yr
yr
yR
Yr
YYRR
YYRr
YYRr
YYrr
YyRR
YyRR YyRr
YyRr yyrryyRr
yyRryyRRYyRr
Yyrr
Yyrr
YyRr
Non-Mendelian Inheritance• Multiple Alleles
– Example: Eye colour in Drosophelia
– Wild Type > Apricot > Honey > White
• Incomplete Dominance– Example: Snap dragons, red X white => pink
• Co-Dominance– Example: Shorthorn cattle, red X white => roan
CR
CR
CW
CRCW
CRCW
CWCW
CRCR
CW
E2
E1
E3
E2E4
E1E3
E3E4
E1E2
E4
Hr
Hr
Hw
HrHw
HrHw
HwHw
HrHr
Hw
• Polygenic Inheritance– Gene interaction: Example Chicken Comb
Single (rrpp)Rose (R-pp)Pea (rrP-)Walnut (R-P-)
• Pleiotropic genes– One gene with multiple effects– Example: Sickle-cell anemia
• Lethal Genes– When a specific genotype is not viable– eg. if AA is lethal, a heterozygous cross would
give a 2:1 ratio (2 Aa : 1 aa – the AA is dead)
Linkage
• Mendel demonstrated his Law of Independent Assortment using simple traits.
• Some traits do not follow this Law.
• When genes are on the same chromosome they do not separate independently