Study of heredity
What is Genetics?
• Heredity: passing of traits
from parent to child
• Patterns of relatedness can
help predict offspring
characteristics
–
–
–
–
–
Autosomal recessive
Autosomal dominance
Sex-Linked
Incomplete dominance
Codominance
• Genes are located on chromosomes
• Genes inherited from mom and dad
• Genes come in several forms called alleles
– Allele: Alternative forms
of a gene
– Ex: Flower color
• White allele or Purple
allele
• An allele is any alternative form of a gene
occurring at a specific locus on a chromosome.
– Each parent donates
one allele for every
gene.
– Homozygous
describes two alleles
that are the same at
a specific locus.
– Heterozygous
describes two alleles
that are different at a
specific locus.
Alleles can be represented using letters. Ex: Aa or AA or aa
– A dominant allele is
expressed as a phenotype
when at least one allele is
dominant.
– A recessive allele is
expressed as a phenotype
only when two copies are
present.
– Dominant alleles are
represented by UPPERCASE
letters
– Recessive alleles by
lowercase letters.
Dominant and Recessive Genes
• Gene that prevents the other gene from “showing” –
dominant
• Gene that does NOT “show” even though it is present –
recessive
• Symbol – Dominant gene – upper case letter – T
Recessive gene – lower case letter – t
Dominant
color
Recessive
color
Genotype and Phenotype
• Combination of genes an organism has (actual gene
makeup) – GENOTYPE
Ex: TT, Tt, tt
• Physical appearance resulting from gene make-up –
PHENOTYPE
Ex: hitchhiker’s thumb or straight thumb
Genetics Vocabulary Review
Choices:
1) YY
2) Tall
3) Yy
4) SS
5) yY
6) gg
7) Smooth
8) Green
9) TT
10) Dwarf
Which choice(s) are examples of:
Genotypes?
1, 3, 4, 5, 6, 9
Phenotypes?
2, 7, 8, 10
Homozygous genotypes?
1, 4, 6, 9
Heterozygous genotypes?
3, 5
Homozygous recessive genotypes?
6
Homozygous dominant genotypes?
1, 4, 9
#1
Dominant vs. Recessive
• If Brown eye color is dominant over blue
eye color, how would you indicate…
– The brown allele?
– The blue allele?
Dominant vs. Recessive
#2
• If dimples in the cheeks are dominant, how would you
indicate…
– The allele for dimples?
– The allele for no dimples?
Dominant vs. Recessive
#3
• If left thumb over right thumb is dominant,
how would you indicate…
– Left over right?
– Right over left?
Mendel laid the groundwork for genetics.
• Traits are distinguishing
characteristics that are
inherited.
• Genetics is the study of
biological inheritance
patterns and variation.
• Gregor Mendel showed
that traits are inherited as
discrete units.
• Many in Mendel’s day
thought traits were
Mendel’s data revealed patterns of
inheritance.
• Mendel made three key decisions in his
experiments.
– use of purebred plants
– control over breeding
– observation of seven
“either-or” traits
• Mendel used pollen to fertilize selected pea plants.
– P generation crossed to produce F1 generation
– interrupted the self-pollination process by removing male
flower parts
Mendel controlled the
fertilization of his pea plants
by removing the male parts,
or stamens.
He then fertilized the female
part, or pistil, with pollen from
a different pea plant.
• Mendel allowed the resulting plants to self-pollinate.
– Among the F1 generation, all plants had purple flowers
– F1 plants are all heterozygous
– Among the F2 generation, some plants had purple
flowers and some had white
• Mendel’s Experiment
Each trait had a 3:1
ratio. No way that
can be a
coincidence!
Mendel’s Findings in Modern
Terms
• If the two alleles of a particular gene present in an
individual are the same, the individual is said to be
homozygous. (dominant or recessive)
• If the alleles of a particular gene present in an
individual are different, the individual is
heterozygous.
• In heterozygous individuals, only the dominant
allele is expressed; the recessive allele is present but
unexpressed.
• Mendel drew three important conclusions.
– Traits are inherited as discrete units.
– Organisms inherit two copies of each gene, one from
each parent.
– The two copies segregate
during gamete formation.
– The last two conclusions are
called the law of segregation.
purple
white
Punnett
Squares
• Defined: Tools used to
determine genetic
probability
• Probability = likelihood
that a certain event will
happen
• 2 Steps:
– 1) Place parent genotypes
on the outside of the box
– 2) Fill in the boxes
Cross a homozygous dominant yellow (YY) plant with
a homozygous recessive green (yy) plant
Key
Y = yellow
y = green
100%
Probability of growing a yellow plant? _____________
0%
Probability of growing a homozygous recessive plant? ____________
0%
Probability of growing a homozygous dominant plant? ____________
Cross a heterozygous dominant yellow (Yy) plant with a
heterozygous dominant yellow (Yy) plant
Key
Y = Yellow
y = green
50%
Probability of growing a heterozygous plant? ________
25%
Probability of growing a green plant? ________
25%
Probability of growing pure yellow? _________
Maria is a heterozygous healthy (Hh) female and Jeff
suffers from recessive cystic fibrosis (hh).
Key
H = healthy
h = cystic
fibrosis
Probability of having a healthy
50%
child? _________
Probability of having a homozygous
0%
dominant child? ________
Autosomal Dominance Inheritance
What’s an Autosome?
• Autosome: refers to chromosomes 1-22
– Ex: Autosomal disorders: gene for the disease is found on
chromosomes 1-22
• Autosomal Recessive Inheritance
– Must inherit two copies of the disorder to be affected
– Healthy is dominant (HH or Hh)
– Disease is recessive (hh)
– Ex: Cystic fibrosis, PKU, Albinism, Sickle cell anemia
• Autosomal Dominance Inheritance
– Only need to inherit one copy of the disorder to be affected
– Disease is dominant (HH or Hh)
– Healthy is recessive (hh)
– Ex: Familial hypercholesterolemia (also called FH), Huntington’s
disease, Neurofibromatosis
Autosomal Dominance Inheritance
• Disease is dominant (FF or Ff)
– Homozygous dominant: early death and don’t survive to reproduce
– Heterozygous live into adulthood
• Healthy is recessive (ff)
• ex: Paul has familial hypercholesterolemia and Stacy is healthy. The
two have 3 children. After testing, the middle child is the only healthy
child.
Ff
ff
Paul
Stacy
Key
disease
disease
F = FH disease
f = healthy
healthy
healthy
Ff
ff
Ff
Autosomal Dominance Inheritance
• Huntington’s disease is a dominant disorder found on chromosome 4.
Betty and Marcus met at a support clinic they have been attending to
help them cope with the knowledge of their illness with Huntington’s
disease. They would like to know the risk of having a healthy child,
now that Betty is pregnant.
Hh
Hh
Marcus
Betty
Key
disease
disease
H = Huntington’s disease
h = healthy
disease
healthy
?