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Transcript 
GENETICS IN A
NUTSHELL
BY: LINDA
HEIDENRICH
BIOLOGY
THE BASICS




Dominant: represented by a large
letter; trait that is expressed
Recessive: represented by a small
letter; trait that is masked unless in
the homozygous form
Genotype: genes in an organism
Phenotype: outer appearance from
the genes
MORE BASICS


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AA=homozygous dominant
Aa=heterozygous
aa=homozygous recessive
AA or Aa=dominant trait
aa=recessive trait
Autosomal Trait

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An autosome is a gene found in the body
cells, i.e. not found on sex chromosomes.
Genetics follow basic heredity rules
regarding dominance and recessiveness.
EXAMPLE: Mendel’s peas
Combine TT (homozygous tall) and Tt
(heterozygous tall)…What do you get?
Welcome the Punnett Square

A Punnett Square explains the
possible outcomes for a mating.
T
T
T
TT
TT
t
Tt
Tt
So…

A cross between a homozygous tall and a
heterozygous tall would yield…
– 50% probability of getting homozygous tall
plants as offspring
– 50% probability of getting heterozygous tall
plants as offspring

HOW do you get a short plant?
– Think, Pair, Share
Well…


Either cross tt x tt or Tt and tt
Try the Punnett Square for each…

T
t
t
t
Sex-Linked



Trait on a gene found on X or Y
chromosome
Usually are recessive disordersapparent only in the homozygous
recessive form
Common example: red-green
colorblindness in boys or hemophilia
Sex-Linked Trait Notation

Notation:
–
–
–
–
–
Male with trait: XhY
Male without trait: XHY
Female with trait: XhXh
Female without trait: XHXH
Carrier female: XHXh

TRY ONE: Cross a hemophiliac male and a carrier
female.
Incomplete Dominance


Autosomal traits BUT the heterozygous
phenotype is a combination of the two
homozygous phenotypes
Example: Color in snapdragons
– RR=red
– Rr=pink
– rr=white

TRY ONE: Cross a pink snapdragon with a red
flower.
Co-dominance


Autosomal traits BUT the
heterozygous is BOTH homozygous
traits. NO RECESSIVE
Example: Horse Coloring
– RR=Red
– RR’=Red/White (Roan)
– R’R’=White

TRY ONE: Cross a two roan horses.
Blood Types

Combination of Co-dominance and
dominant and recessive patterns
– AB blood: IA IB
– A blood: IA IA or IA i
– B blood: IB IB or IB i
– O blood: ii

TRY ONE: Cross a heterozygous A blood
female with a homozygous B blood male.
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