HEREDITY
Heredity: the passing of traits from parents to offspring.
Trait: A physical or behavioral characteristic that describes
an organism (Ex: height; eye color; endurance)
Importance of Gregor Mendel
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Austrian botanist monk
Mid 1800s
Formed the basic laws of
heredity
His work laid the foundation to
the study of heredity
Mendel is referred to as “The
Father of Genetics.”
Genetics = study of genes,
heredity, and genetic variation.
Mendel’s Pea Plants
• Mendel studied garden pea plants.
• Observed differences in multiple traits over
many generations, because pea plants reproduce
rapidly, and have many visible traits such as:
Plant Height
Seed Color
Green
Green Yellow
Tall
Short
Pod color
Yellow
Seed Shape
Pod Shape
Wrinkled Round
Smooth Pinched
Mendel’s Experiments
Noticed that some plants always produced offspring that had a form of
a trait exactly like the parent plant (“purebred”)
Example: purebred short plants always produced short offspring
and purebred tall plants always produced tall offspring.
X
Purebred Short Parents
Short Offspring
X
Purebred Tall Parents
Tall Offspring
Mendel’s First Experiment
Mendel crossed purebred plants with opposite forms of a trait.
Example: Purebred tall plants were crossed with purebred short plants.
X
Parent Tall
P generation
Parent Short
P generation
Offspring Tall
F1 generation
Mendel observed that all of the offspring grew to be tall plants. None
resembled the short parent.
P stands for “parent” and F# stands for “filial” (son).
Mendel’s Second Experiment
Mendel then crossed two of the offspring tall plants produced from his
first experiment.
Parent Plants
Offspring
X
Tall
F1 generation
F2 generation: 3⁄4 Tall & 1⁄4 Short
To his surprise, Mendel observed that this generation had a mix of tall
and short plants, even though none of the F1 parents were short!
Mendel’s Law of Segregation
Mendel’s first law, the Law of Segregation, has three parts.
From his experiments, Mendel concluded that:
1. Traits are handed down through “hereditary factors”
in the sperm and egg.
2. Because offspring obtain hereditary factors from both
parents, each organism must contain two factors for every
trait.
3. The factors in a pair segregate (separate) during the
formation of sex cells, and each sperm or egg receives
only one member of the pair.
Dominant and Recessive Alleles
One factor (gene) in a pair may mask,
or hide, the other factor.
Example: When he crossed a
purebred tall plant with a purebred
short plant, all offspring were tall
even though all the F1 offspring
should have both tall and short
factors. (The tallness factor masked
the shortness factor.)
Dominant and Recessive Alleles
Genes: a section of DNA that determines a trait.
Alleles: the different forms of a gene.
Alleles are abbreviated with a letter.
Example:
T = allele for “tall” height
t = allele for “short” height
Every gene has two alleles (1 from mom & 1 from dad).
Dominant and Recessive Alleles
Alternative versions of genes (different alleles) result in variations in
inherited traits.
Example: The gene for flower color in pea plants exists in two
versions, one for purple flowers and the other for white flowers
Each gene resides at a specific location on a specific chromosome
(remember that there are multiple chromosomes)
Dominant and Recessive Alleles
If the two alleles for a gene are different, the dominant allele will
determine the organism’s appearance and “mask” the other allele.
Dominant alleles are shown with a Capital letter.
Dominant alleles are “STRONGER.”
The recessive allele will only determine the organism’s appearance if
both alleles on the gene are recessive.
Recessive alleles are shown with a lower-case letter.
Recessive alleles are “weaker.”
Dominant Alleles
Mendel observed a variety of dominant alleles in pea plants other than
the tall allele. For instance, a plant that is heterozygous for seed
color will always have yellow seeds.
Green & Yellow Allele
Yellow Seed
However, a plant that is heterozygous for pod color always displays
the green allele.
Green & Yellow Allele
Green Pod
In addition, round seeds are dominant over wrinkled seeds, and
smooth pods are dominant over wrinkled pods.
Homozygous Genes
We know now that Mendel’s “purebred” plants had two identical
alleles for a particular gene. The modern scientific term for “purebred”
is homozygous.
Homozygous: having the same allele for a particular gene (NN or nn).
Example:
A homozygous tall plant
has two tall alleles (TT)
and will only contribute a
tall allele (T).
A homozygous short plant
has two short alleles (tt)
and will only contribute a
short allele (t).
Heterozygous Genes
When the two types of homozygous plants were crossed the offspring
received one tall gene and one short gene from the parent plants.
All offspring contained both alleles (1 tall allele and 1 short allele).
Heterozygous: possessing two different forms of a gene, one inherited
by each parent (Nn).
Although the offspring
have both a tall and a short
allele, only the tall allele is
expressed (the tall allele is
dominant and “masks” the
short allele).
Genotype & Phenotype
Genotype: the two alleles that an organism has for a trait.
(Example: AA, Aa, aa)
Phenotype: the physical trait that the organism shows or “expresses”.
(Example: Does the butterfly look dark or light in color?)
Law of Independent Assortment
Each pair of genes separate independently of each
other in the production of sex cells (egg or sperm).
Example: Each sex cell will have
only 1 allele for every gene
(haploid). The plant will donate
either a yellow or green seed allele,
or a wrinkled or a smooth pod
shape. If the plant donates the
yellow seed allele it does not mean
that it will also donate the wrinkled
pod allele.
The donation of one allele
from each pair is
independent of any other
pair.
Remember:
1) There are 2 alleles for every gene.
2) Dominant alleles are “stronger” and Capitalized.
3) Recessive alleles are “weaker” and lower-case.
4) Recessive traits will only be shown if both alleles are recessive.
What are the possible allele combinations for these 2 plants?