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Genetics
Study of heredity
Heredity is the study of how offspring get
their traits from their parents.
Traits are physical characteristics:
Height, hair color, flower color
Where do traits come from?
Traits are controlled by genes.
Genes are found on chromosomes
Chromosomes are wound up DNA
Chromosomes
Humans have 46 chromosomes
They are organized into 23 pairs
Chromosomes and Genes
Every chromosome contains thousands of
genes
Each gene has 2 parts
These parts are called alleles
One allele comes
from each parent
Alleles
There are two kinds of alleles
Dominant and recessive
Dominant alleles are more powerful
They can hide recessive alleles when paired
together.
DOMINANT ALLELES ARE SHOWN
WITH CAPITAL LETTERS
recessive alleles are shown with lower
case letters
Alleles and Traits
The combination of alleles an organism
has determines what it will look like.
2 capital alleles (QQ) will show the dominant trait
2 lower case alleles (qq) will show the recessive
trait
1 dominant and 1 recessive allele (Qq)
combination will show the dominant trait
How does this work?
Each parent give the offspring 1 allele.
q
Q
However…
Each parent has 2 possible alleles to give.
But only one will be given to their offspring
Q
q
Q
q
Parents and Offspring
The alleles the parents can give the
offspring determine what the offspring will
be.
Offspring possibilities
The alleles given by the parents will
determine what the offspring has.
Q
+
q
=
Qq
Defining allele combinations
If the alleles given to the offspring by the
parents are the different it is called
heterozygous or hybrid.
Q
q
+
+
q
Q
=
Qq
=
Qq
Defining allele combinations
If the alleles given to the offspring by the
parents are the same it is called
homozygous or purebred.
Q
q
+
+
Q
q
=
QQ
=
qq
Types of Homozygous/purebred
combinations
If the 2 alleles given by the parents are
dominant alleles it’s called homozygous
dominant or a dominant purebred.
Q
+
Q
=
QQ
Types of Homozygous/purebred
combinations
If the 2 alleles given by the parents are
recessive alleles it’s called homozygous
recessive or a recessive purebred.
q
+
q
=
qq
Genes and physical appearance
The genetic code found on the
chromosomes is referred to as the
genotype.
QQ, Qq, qq
The physical trait that results is referred to
as the phenotype.
Predicting the possible offspring
The tool geneticists use to predict the
possible offspring is called a Punnett
Square
Reading a Punnett Square
A Punnett square show the probability that
parents will have certain offspring.
Phenotypes:
•75% chance the offspring will be tall
•25% chance the offspring will be short
Genotypes:
•25% chance the offspring will be
homozygous dominant/dominant purebred
•25%chance the offspring will be
homozygous recessive/recessive purebred
•50% chance the offspring will be
heterozygous/hybrid