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BASIC
GENETICS

GENE: a unit of DNA that controls a trait of a
particular organism (ex. Freckles)
Homologous Chromosomes
Allele #1
Allele #2
Gene

ALLELE: an alternative form a gene (one
member of a pair) that is located at a specific
position on a specific chromosome.
DOMINANT allele
An allele that can “mask over” a
recessive allele. Only the
dominant trait is expressed.
It is generally represented by a
CAPTIAL letter
RECESSIVE allele
An allele that can only be
expressed if a dominant allele is
absent.
It is generally represented by a
LOWER CASE letter
Ex. “b”
Ex. “B”
HOMOZYGOUS
(purebred)
HETEROZYGOUS
(hybrid)
A term used to describe a pair
of alleles that are IDENTICAL
or the SAME.
A term used to describe a pair
of alleles that are DIFFERENT.
Ex. BB or bb
Ex. Bb
Homologous Chromosomes
Homologous Chromosomes
Gene
Gene
GENOTYPE
PHENOTYPE
The description of the gene pair. The observable trait that results
from the action of an allele pair.
Ex. BB, Bb, bb
Ex. Brown eyes or blue eyes
Genotype(s):
Phenotype:
Genotype:
Phenotype:
HOW TO SET UP YOUR PUNNETT SQUARE
1.
2.
3.
4.
5.
6.
Read the word problem. Identify the dominant
and recessive traits.
Make a key to represent the dominant and
recessive alleles.
Determine the alleles of the parents.
Place the alleles of the parents along the top
and left side of the punnett square.
Cross the alleles inside the boxes.
Determine the ratios of the offspring.
CROSS A HOMOZYGOUS PURPLE FLOWER WITH A
WHITE FLOWER. PURPLE IS DOMINANT TO
WHITE.
PHENOTYPIC
RATIO:
GENOTYPIC
RATIO:
CROSS TWO HETEROZYGOUS BROWN HAIRED
PARENTS. BROWN IS DOMINANT TO BLONDE.
PHENOTYPIC
RATIO:
GENOTYPIC
RATIO:
A HETEROZYGOUS TALL PLANT IS CROSSED
WITH A SHORT PLANT. TALL IS DOMINANT TO
SHORT.
PHENOTYPIC
RATIO:
GENOTYPIC
RATIO:
PEDIGREES
PEDIGREE ANALYSIS
Male =
Sibling =
Female =
Fraternal Twins =
Marriage =
Identical Twins =
(Non-identical)
Organization Rules:
1. Generations are numbered using Roman numerals, I, II,
III, etc.
2. Children are placed in birth order starting from the left
side.
KEY
= healthy
= Tay Sacs
PEDIGREE
KEY
= brown eyes
= blue eyes
PEDIGREE
KEY
Brown Hair (B) =
Blonde hair (b) =
The O’Connor Pedigree for Brown Hair
Family Pedigree for Poison Ivy Susceptibility
KEY
Immune to poison ivy (R) =
Susceptible to poison ivy (r) =
I.
II.
III.
KEY
Six-fingered (F) =
Five-fingered (f) =
Family Pedigree for Polydactly
COMPLEX
GENETICS
INCOMPLETE DOMINANCE

INCOMPLETE DOMINANCE:
A type of inheritance when a heterozygous
geneotype expresses a blending phenotype.
R= red
W= white
#1 Cross a snapdragon white flower with a pink flower.
R= red
W= white
#2 Cross a snapdragon pink flower with a pink flower.
C = curly hair
S = straight hair
#3 Cross a wavy haired person with a straight haired person.
COMPLEX
GENETICS
CO - DOMINANCE

Co-Dominance: 2 alleles are equally dominant
and expressed.

There are 3 alleles for blood typing: A, B, O
A and B alleles are co-dominant
 O allele is recessive
• Even though “O” is capitalized, it is still considered a
recessive allele.

PHENOTYPE
GENOTYPE
“A” blood
AA, AO
“B” blood
BB, BO
“AB” blood
AB
“O” blood
OO
#1 CROSS A PERSON WITH HOMOZYGOUS “A” BLOOD AND
A PERSON WITH HOMOZYGOUS “B” BLOOD.
#2 CROSS AN “O” BLOOD PERSON WITH A “AB”
BLOOD PERSON.
#3 CROSS A HETEROZYGOUS “A” BLOOD PERSON
WITH A HETEROZYGOUS “B” BLOOD PERSON.
COMPLEX
GENETICS
SEX-LINKED
SEX-LINKED INHERITANCE
A type of inheritance where certain alleles are found
only on one sex chromosome (X)
Color Blindness
Hemophilia
Werewolf
Syndrome
TEST# 1
TEST# 2
RED-GREEN COLOR BLINDNESS
GREEN-YELLOW COLOR BLINDNESS
BASIC SEX DETERMINATION PUNNETT
SQUARE
PHENOTYPE
GENOTYPE
Normal Female
XBXB
Normal Female
XBXb (carrier)
Abnormal Female
XbXb
Normal Male
XBY
Abnormal Male
XbY
#1 CROSS A COLORBLIND MALE WITH A NORMAL
FEMALE.
#2 CROSS A COLORBLIND MALE WITH A NORMAL
FEMALE CARRIER.
KEY
Normal =
Colorblind =