genetics. terminology allele – contrasting form of a gene –ex: t = tall; t = short –ex: g =...
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Genetics
Terminology
• Allele – contrasting form of a gene– Ex: T = tall; t = short– Ex: G = green; g =
yellow– CAPITAL LETTERS –
DOMINANT TRAIT– lowercase letters –
recessive trait
More Terms
• Genotype– Genetic makeup of an organism
• Phenotype– Physical appearance
More Terms
• Homozygous Alleles– when the alleles of a
pair are the same– Ex: GG, TT, rr, nn
• Heterozygous Alleles– when the alleles are
not the same– Ex: Gg, Tt, Rr, Nn
Probability
• The likelihood that a specific event will occur
Probability = -----------------------------# of one kind of event
# of all events
Coin Toss
• Probability of a coin landing heads up?
• The more times you flip the coin, the closer to 50% the probability will get
Punnett Squares
• Device used in predicting possible offspring
Monohybrid Cross
• A cross that involves only one trait with two phenotypes
• Ex: Seed color– GG x gg;– G = green, g = yellow
Making a Punnett Square
• Draw a box
• Put one set of traits on top– Male parent
• Put the other on the side– Female parent
Parent Generation: P1 GG x gg
G G
g
g
Gg
Gg
Gg
Gg
Cross F1 Generation
• Gg x Gg
• Genotype?
• Phenotype?
Parent Generation: F1 Gg x Gg
G g
g
G GG
Gg
Gg
gg
Results
• Genotypes
– 25% GG
– 25% gg
– 50% Gg
• Phenotypes
– 75% green
– 25% yellow
GG Gg
Gg gg
G
G
g
g
You Try
•TT x Tt (T = tall; t = short)
TT x Tt (T = tall; t = short)
T T
t
T TT
TtTt
TT
• Genotypes– 50% TT– 50% Tt– 1:1
• Phenotypes– 100% tall
• Bb x bb (B = black; b = white)
You TryYou Try
Bb x bb (B = black; b = white)
• Genotypes– 50% Bb– 50% bb– 1:1
• Phenotypes– 50% black– 50% white– 1:1
B
Bb
b
b
b bb
bbBb
• A cross between a homozygous recessive individual and an unknown
• Used to determine whether an individual is homozygous dominant or heterozygous
Testcross
Testcross
• Mendel developed what is known as a test cross.
• He took a homozygous recessive individual and mated it with a pea plant showing the dominant trait.
bb x Bb or BB
Testcross• If in the F1 generation all the offspring
showed the dominant trait, then the dominant parent was probably BB.
b b
B
B?
Bb Bb
BbBb
Testcross
• If there were any recessives in the F1, then the dominant parent had to be Bb.
bb
B
b?
Bb Bb
bb bb
What if we are looking at two traits at a time?
What if we are looking
at Tall plants with Axial flowers
(TTAA) and cross it with a short plant with terminal
flowers (ttaa)?
T
If we look at meiosis what does that tell us?
T
A
A T
T
A
AParent TTAA
• Parent TTAA can only pass on the alleles
TA to the offspring
• What about Parent ttaa?
• Can only pass on the alleles ta to the offspring
TA
ta TtAa
Parents: TTAA x ttaa
Parent TTAA produces only one kind of gamete.
Parent ttaa produces only one kind of gamete.
Thus our punnett square is very simple.
What about the F1 generation?
T
t a
tA
t
T
T
a
A
A
aParent TtAa
TA tatATa
TA
Ta
tA
ta
TTAA
ttaa
TTAa
TTAa
Ttaa
TtAA
TtAa
ttAa
TtAaTTaa
ttAa
TtAA
TtAa
TtAa
Ttaa
ttAA
F1 generation: TtAa x TtAa
TA
• A cross with two traits
• Ex: seed color & seed shape– R = round, r = wrinkled; Y = yellow, y = green– RRYY x rryy– YYRr x yyRR
Dihybrid Cross
You try
RRYY x rryyR = round, r = wrinkled; Y = yellow, y = green
Dihybrid Cross
Parents: RRYY x rryy
RrYy
RY
ry
All offspring will be heterozygous
You try
F1 generation: RrYy x RrYyR = round, r = wrinkled; Y = yellow, y = green
Dihybrid Cross
F1 generation RrYy x RrYy
• Phenotypes– Round: Yellow– Round: Green– Wrinkled: Yellow– Wrinkled: Green
– 9:3:3:1 ratio
F1 generation: RrYy x RrYy
93
3
1
Sex-linked Genes
• Genes carried on the X and Y chromosomes are called sex-linked genes.
• Traits that are controlled by these genes occur more often in one sex than the other.
• Can you explain why?
• Colorblindness is one example of a trait controlled by sex-linked genes.
Sex-linked Genes
Color blindness
• The normal human retina's color receptors are tuned to green, blue, and red. Working together, the three give us our colorful view of the world. When one or more of those color receptors is missing the result is color-blindness. The genes for our red and green color receptors are located on the X-chromosome, giving women a redundant set of receptor genes. This is why men are far more prone to color-blindness than women.
XCY x XX
Sex-linked Genes
Recessive gene for color blindness
Sex-linked Genes
XC Y
X
X
XCX
XCX
XY
XY