Download Genetics Notes

I.
Genetics
A. Gregor Mendel
1. Known as the father of genetics.
2. Worked with garden peas.
3. Led to knowledge of inheritance.
4. Heredity – transmission of traits
from parents to the offspring.
B. Gene – the basic unit of inheritance.
1. Made of DNA.
2. In sexual reproduction:
a) One gene from the male parent.
b) One gene from the female parent.
c) Why you have two copies of same
chromosome.
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3. Alleles – alternate forms of a gene.
a)Ex: seed color (yellow, green seed)
C. Dominant and Recessive alleles
1. Dominant alleles – an allele that masks
the presence of another allele.
a)Always use capital letters.
2. Recessive alleles – an allele that is
being masked by the dominant allele.
a)Always use lower-case letters.
3. Homozygous – when both alleles are the
same (homo = same).
a)Homozygous Dominant (AA) or
homozygous recessive (aa)
4. Heterozygous – when both alleles are
NOT the same (Hetero = different)
a)Heterozygous dominant (Aa)
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5. Genotype – genetic makeup of an
organism.
a) Ex: AA, Aa, aa
b) Geno = genes, genetics
6. Phenotype – physical appearance of an
organism.
a) Ex: tall, yellow, wrinkled
b) Pheno = physical
II. Mendel’s work
A. Three Principles:
1. Principle of dominance – an allele
masks another allele.
2. Principle of Segregation – two
alleles for a characteristics
separate during gamete formation. 3
3. Principle of Independent Assortment –
alleles for different characteristics
(genes) are distributed to gametes
independently.
III. Punnett Squares – used to describe the
ratio of expected outcomes of a genetic
cross.
A. Monohybrid cross – (mono meaning one) 1
trait is crossed.
1. Have two alleles for each gene.
2. Ex:
T = tall, dom
t = short
TT x tt
(parents)
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3. More ex:
TT x Tt
Tt x Tt
4. Testcross – an individual with unknown
genotype is crossed with a homozygous
recessive individual.
a)Ex: rabbit color
B = black, dom
b = brown
BB x bb
Bb x bb
• If any brown rabbits then the parents
were heterozygous, if not then they
were homozygous.
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5. Incomplete dominance – when one allele
does not completely mask another.
a)The organism shows a blend of the
two.
b)Ex: Japanese 4 o’clocks R = red
W = white
RR x WW
RW = pink
6. Codominance – when both alleles
contribute to the phenotype of the
organism (no blending).
a)Ex: cattle color
R = red
r = white
RR x Rr
Rr = roan
7. Polygenic traits – traits controlled by
two or more genes. (poly = many)
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8. Sex-linked traits – trait occurs only
on a sex chromosome.
a)Ex: colorblindness
XX = female
B = normal, dom
XY = male
b = colorblindness
XBXB = normal female
XBXb = normal female, carrier
XbXb = colorblind female
XBY = normal male
XbY = colorblind male
XBXb x XBY
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9. Blood types:
a)A blood = IAIA or IAi
b)B blood = IBIB or IBi
c)AB blood = IAIB
d)O blood = ii
B. Dihybrid Crosses – two genes are crossed
at the same time.
1. Ex: seed color and texture
P = purple kernel, dom
P = yellow kernel, rec
S = smooth, dom
S = wrinkled
PpSs x PpSs
(remember Foil – for
all possible combinations of gametes)
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