Genetics Unit
Genetics = the field of biology
devoted to understanding how
characteristics are transmitted
from parents to offspring
Heredity = the transmission of
characteristics from parents to
offspring
Mendel
- “father” of modern genetics = laid
the groundwork
- experimented with garden peas
- looked for traits =
specific
characteristics
Mendel (cont)
- P1 generation = parental
generation
- F1 generation = offspring of the
parental generation
- F2 generation = offspring of the
F1 generation
Mendel (cont)
Mendel’s 1st Law = Law of Segregation
1) Organisms inherit two copies of
each gene, one from each parent
2) Organisms donate one copy of
each gene in their gametes.
Mendel (cont)
Mendel’s 2nd Law = Law of
Independent Assortment
1) The presence of one trait does not
affect the appearance of another
trait
Basics of Genetics
- Gene = a segment of DNA on a
chromosome that controls a specific
trait
- because chromosomes come in
pairs, genes come in pairs
Basics of Genetics (cont)
- Allele = each of several forms of a
gene
- aka: letters
- each allele has a specific location
on a chromosome (= locus)
- capital letters = dominant alleles
- lowercase letters = recessive
alleles
Basics of Genetics (cont)
- Homozygous = both alleles are alike
- homozygous dominant = both
capital letters = BB
- homozygous recessive = both
lowercase letters = bb
- Heterozygous = alleles are different
= Bb
Polydactyly
Polydactyly is the
condition of
having more than
the typical
number of fingers
or toes. The allele
for polydactyly is
dominant.
Basics of Genetics (cont)
- Genotype = genetic makeup of an
organism
- consists of alleles (letters)
- Phenotype = the appearance of an
organism as a result of its genotype
- aka: what does it look like
- human phenotype can be altered
by behavior
Monohybrid Crosses
= a cross between individuals
that involves one pair of traits
- Example: Black hair (B) in
guinea pigs is dominant to
brown hair (b)
Example 1:
Homozygous Dominant X Homozygous Dominant
Genotypic Ratio
Phenotypic Ratio
Example 2:
Homozygous Recessive X Homozygous Recessive
Genotypic Ratio
Phenotypic Ratio
Example 3:
Homozygous Dominant X Heterozygous
Genotypic Ratio
Phenotypic Ratio
Example 4:
Homozygous Recessive X Heterozygous
Genotypic Ratio
Phenotypic Ratio
Example 5:
Heterozygous X Heterozygous
Genotypic Ratio
Phenotypic Ratio
Testcross
= an individual of unknown
genotype is crossed with a
homozygous recessive
individual
- can be used to determine the
genotype of any phenotype
that is dominant
Example 6: Testcross
Genotypic Ratio
Phenotypic Ratio
Example 6: Testcross
Genotypic Ratio
Phenotypic Ratio
Example 6: Testcross
Genotypic Ratio
Phenotypic Ratio
Incomplete Dominance
= the F1 Generation will have a
phenotype in between that of
the parents
- Example: Red flowers (R) and
White flowers (r) can make
Pink flowers (Rr)
Example 7: Incomplete Dominance
Pink Flower X Pink Flower
Genotypic Ratio
Phenotypic Ratio
Example 7: Incomplete Dominance
Pink Flower X Red Flower
Genotypic Ratio
Phenotypic Ratio
Example 7: Incomplete Dominance
White Flower X Red Flower
Genotypic Ratio
Phenotypic Ratio
Example 7: Incomplete Dominance
Pink Flower X White Flower
Genotypic Ratio
Phenotypic Ratio
Codominance
= when both alleles for a gene
are expressed in a
heterozygous offspring
- Example: Red coat color (R)
in horses is codominant with
’
white coat color (R ) to make a
horse with a mix of red and
’
white coat color (RR )
Example 8: Codominance
Red Coat X White Coat
Genotypic Ratio
Phenotypic Ratio
Example 8: Codominance
Red/White Coat X White Coat
Genotypic Ratio
Phenotypic Ratio
Example 8: Codominance
Red/White Coat X Red Coat
Genotypic Ratio
Phenotypic Ratio
Dihybrid Cross
= a cross between individuals
that involves two pairs of traits
- Example: Black hair (B) in
guinea pigs is dominant to
brown hair (b) and rough coat
(R) is dominant to smooth
coat (r)
Example 1:
Homozygous Dominant X
Homozygous Recessive
Example 1:
Homozygous Dominant X Homozygous Recessive
Example 2:
Heterozygous X
Heterozygous
Example 2:
Heterozygous X Heterozygous