GENETICS CHEAT SHEET
Homozygous= pure bred
Heterozygous = hybrid = carrier
Genotype: Two letters, meaning the two alleles ( forms of a gene) for each characteristic.
Example: AA, Aa, or aa.
Phenotype: Expression of the genotype (how the genes look like).
attached earlobes, unattached earlobes.
Example:
1. Dominant/Recessive (Mendelian Genetics)
Dominant always shows. Recessive only shows if dominant is not present.
In Autosomal chromosomes. The gender does not matter for these characteristics.
Example:
A
a
AA: Unattached ear lobe (homozygous dominant)
a
Aa
aa
Aa: Unattached ear lobes (heterozygous)
a
Aa
aa
aa: Attached ear lobes (homozygous recessive)
50% Unattached (Aa), 50% attached (aa).
2. Incomplete Dominance:
The heterozygous is a MIX between the homozygous dominant and the homozygous
recessive.
In Autosomal chromosomes. The gender does not matter for these characteristics.
Example:
RR: Red color
R
r
R
RR
Rr
r
Rr
rr
rr: white color
Rr: pink color
25% red (RR), 25% white (rr), 50% pink (Rr)
3. Codominance:
Both alleles are dominant. Both alleles show in heterozygous.
In Autosomal chromosomes. The gender does not matter for these characteristics.
Example:
FWFW: White feathers
FW
FW
FB
FWFB
FWFB
FB
FWFB
FWFB
FBFB: Black feathers
FWFB: White and black feathers.
100% Black and white feathers (FWFB)
4. Blood Types:
In Autosomal chromosomes. The gender does not matter for these characteristics.
Phenotype:
Genotype:
A
IAIA (homozygous) or
IA i (heterozygous)
B
IBIB
I Bi
AB
IAIB (codominant)
O
ii (hozygous recessive)
(homozygous) or
(heterozygous)
IA
i
IB
IAIB
IBi
i
IA i
ii
25% AB (IAIB), 25% B (IBi ), 25% A (IA i ), 25% 0 (ii).
5. Sex-linked traits:
These traits are recessive (lower case letter).
Traits related to chromosome 23 (X). The gender matters!!
XX: women; XY: men
XA
Xa
Xa
XAXa
X aX a
Y
XAY
X aY
Example: Color vision:
XAXA
XaXa
Women:
(normal vision, dominant homozygous),
(color
blind, recessive homozygous, or XAXa (heterozygous= Carrier).
Men: XAY (normal color vision), or XaY (color blind)
Girls: 50% carriers (XAXa ), 50% color blind (XaXa ).
Boys: 50% normal vision (XAY), 50% color blind (XaXa)