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Genetics-Heredity
Ch 19 Holt
Ch 15 McDougal
Genetics, Heredity, & Traits
• Genetics is the study of how traits are
passed from parent to offspring
• Heredity occurs when traits are passed
from the parents to their offspring.
• Traits are different forms of physical
characteristics that an organism inherit like
black hair, brown eyes, or brown skin.
• Can you list 4 of your traits?
Characteristics v. traits
• Characteristics are physical features that
an organism can have.
• Traits are different forms of a
characteristic.
Characteristic
Traits
Brown eyes
Eye Color
Blue eyes
hazel eyes
Fill in the missing information from the diagrams below:
Characteristic
Traits
Straight hair
Characteristic
Traits
oval
Hair Texture
Curly Hair
Wavy hair
Eye Shape
ellipse
circle
Gregor Mendel
• Mendel was an Austrian monk who
studied pea plants.
• Mendel noticed that there were patterns to
the inheritance of some traits and wanted
to learn how traits are inherited.
• He predicted that the traits were inherited
in patterns and designed an experiment to
test it.
Characteristics
studied by Mendel
•In Mendel’s experiments, he
crossed true-breeding plants.
•Repeated the experiment
several times with 7 different
characteristics.
•Mendel calculated ratios to
determine the pattern.
•Concluded there was a
pattern: Dominant/Recessive.
•Must be two sets of
instructions controlling
inheritance.
GENES
• It was later discovered that the two sets of
instructions controlling inheritance were
found on segments of DNA called genes.
• Genes are made from DNA and located on
chromosomes.
• DNA- deoxyribonucleic acid: contains the
genetic information that determines what
traits are inherited.
• Two genes for each characteristic is found
in organisms that reproduce sexually. 1
Genes and Alleles
• Genes give the instructions for a trait.
Alleles are different forms of a gene.
S
C
{
Curly hair
allele
Straight hair
Phenotype is the
physical
appearance of an
expressed gene.
Here straight hair
or curly hair would
be phenotypes.
Genes for
hair texture
Genotypes
Genotypes are the two genes passed on for a trait; can be homozygous
or heterozygous.
{
b
Genes for
eye color
Blue Eyes
Blue Eyes
b
allele
Brown Eyes
Blue Eyes
allele
heterozygous
B
b
{
homozygous
Genes for
eye color
Types of Inheritances
Genes can be expressed in many ways to
produce traits.
• Dominant/Recessive
• Incomplete Dominance
• Co-dominance
• Multiple Alleles
• Polygenic Inheritance
Dominant/Recessive
• In dominant/recessive inheritance one
gene covers up or masks the expression
of the other.
• Different forms of a gene is called an allele
– Dominant genes are represented with capital
letters
– Recessive genes are represented with lowercase letters.
Incomplete Dominance
• The phenotype is intermediate to those of
the two parents.
• An example is flower color in the four
o’clock and snapdragon plant.
• In both these plants red and white
homozygous plants are crossed and the
resulting offspring are pink.
Co-dominance
• The phenotype contains both of the traits
observed in both parents
• For instance, in some types of roses, if a
white rose is crossed with a red rose the
offspring shows both red and white colors.
Multiple Alleles
• A trait that is controlled by more than two
alleles.
• Even though many forms of a gene can
exist for a certain trait, organisms can
have only two, one from each parent.
• An example is Blood type in human which
is controlled by three alleles A, B, and O.
Polygenic Inheritance
• A group of gene pairs act together to
produce a single trait.
• An example would be fingerprints, eye
color, height, weight, skin color.
• Each allele contributes to the trait even if
the effect is small. The combination
produces a wide variety in the phenotype.
Sex-Linked Traits
•Traits that are expressed from genes that are
carried on the sex chromosomes: X, Y
chromosomes.
•Men have an XY sex chromosome combination
•Women have XX sex chromosome combination.
Genes carried on the X chromosomes are x-linked
and genes carried on the Y chromosome are ylinked.
•An example is eye color in fruit flies or hairy ears
Punnett Square
• The Punnett Square is a graphic tool
used to predict the results of a genetic
cross.
A
a
Mom’s Alleles
Offspring Genetics
Dad’s Alleles
A
Genotype: AA, Aa, aa
1/4, 2/4, ¼
25%,50%,25%
a
Using a Punnett Square
In garden peas, round seed coats (R) are
dominant over wrinkled seed coats
(r). What will the results be of a cross
between a homozygous dominant male and a
recessive female? What are the chances of
producing a wrinkled seed?
Genotype: homozygous dominant male (RR)
recessive female (rr)
So the cross would have RR x rr as parents
Mom’s Alleles
r
r
Offspring Genetics
Dad’s Alleles
R
Genotype: Rr
Probability: 4/4 or 100%
Phenotype : Round,
100%
R
So what are the chances of these parents producing a wrinkled seed?
Looking at the probable phenotypes the chances are 0% or none.