Heredity & Genetics

Heredity & Genetics

Heredity is the passing on of observable, heritable traits (e.g., eye color,
height, etc.) from parents to offspring from one generation to the next

Genetics is the scientific study of heredity, genes, and genetic variation

Much of what we know about modern genetics started with the research of
Gregor Mendel, a 19th Century Augustinian monk. Mendel studied pea plants
and determined that some traits were dominant (e.g., yellow color) and others
recessive (e.g., green color). This dichotomy between dominant and recessive
heritable traits forms the basic premise of Mendelian genetics

Heredity & Mendelian Genetics

Alleles are different forms of
genes, which are segments of
coding DNA that constitute the
basic functional units of heredity

In Mendelian genetics, dominant
traits are always expressed over
recessive traits, as long as at least
one dominant allele is present;
two copies of the recessive allele
are required for the recessive trait
to be expressed, rather than
suppressed by the dominant trait

Alleles

Dominant alleles are written as
uppercase/capital letters (e.g., B)

Recessive alleles are written as
lowercase/little letters (e.g., b)

Genotypes are combinations of two
(or more) alleles for a given trait;
overall genetic makeup of organism

Phenotypes are the physical
features of an organism resulting
from its genotype (and environment)

Alleles

There are three main genotypes in
Mendelian genetics:

Homozygous dominant (e.g., BB)

Homozygous recessive (e.g., bb)

Heterozygous / Hybrid (e.g., Bb)

Can you identify each genotype in
the Punnett Square on the right?

Genotypes

Punnett squares are diagrams used to
help predict the genotypes and
phenotypes (and their probabilities) of
a test cross/breeding between two
sexually reproducing organisms

Probability is how likely (%) a given
outcome is compared to others

What is the probability for each
genotype in the Punnett square? What
is the probability for each phenotype?

Punnett Squares

The P Generation (parental) is the
starting generation used for a cross

The F1 Generation (fillial) includes
the offspring of the first generation
resulting from the initial cross of
parents in the P Generation

The F2 Generation (fillial) includes
the offspring resulting from a cross
between two individuals from the F1
Generation (e.g., self-fertilization)

Generations

Based on his experiments with pea
plants, Mendel discovered that there
was a 3:1 ratio between dominant and
recessive traits in the F2 Generation

Q: How can white flowers reappear in
the F2 Generation if all the plants in the
F1 Generation have purple flowers?

Generations

Multiple Choice

What is an allele?

• A version of a gene, which is neither dominant nor recessive according to Mendelian genetics.

• A version of a gene, which is always dominant or always recessive according to Mendelian genetics.

• A version of a gene, which can either be dominant or recessive according to Mendelian genetics.

A monohybrid cross is a cross
between two individuals for one trait

We can set up a Punnett square for
each different generation in Mendel’s
experiment as a monohybrid cross

Monohybrid Crosses

Monohybrid Cross Practice

Let’s try a couple of examples:

Rr × Rr

AA × aa

Zz × zz

The results from Mendel’s experiments led
to Mendel’s Law of Segregation

Parental alleles segregate/separate
randomly during anaphase of meiosis, so
each gamete only contains one allele for a
given trait. When two gametes unite and
a zygote is formed, that offspring receives
one allele from each parent. There is an
equal chance (50%) of receiving one of
the two alleles (dominant or recessive)

Mendel’s Law of Segregation

A dihybrid cross is a cross between
two individuals for two traits

We can set up a larger Punnett square
to help us solve a dihybrid cross →

For the F2 Generation, Mendel found
a 9:3:3:1 ratio for phenotypes with two
different traits (color and shape)

9 offspring dominant for both traits, 3
dominant for color, 3 dominant for
shape, 1 recessive for both traits

Dihybrid Crosses

Dihybrid Cross Practice

Let’s try a couple of examples:

RrYy × RrYy

AABb × aabb

PPDD × ppdd

Multiple Choice

Which of the following genotypes is homozygous dominant?

• aa

• aA

• Aa

• AA

Multiple Choice

Which of the following genotypes is homozygous recessive?

• aa

• aA

• Aa

• AA

Multiple Choice

Which of the following genotypes is heterozygous?

• aa

• aA

• Aa

• AA

Multiple Choice

What is the difference between the genotype and phenotype of an organism?

• The genotype includes all of the physical traits, and the phenotype includes all of the genetic material.

• The genotype and phenotype are the same thing in genetics.

• The genotype includes all of the genetic material, and the phenotype includes all of the physical traits.

Multiple Choice

What is the typical phenotype ratio expected for the F2 generation from a dihybrid cross performed on organisms that exhibit Mendelian genetics?

• 9:3:3:1

• 9:6:1:1

• 6:3:3:1

• 3:1

Mendel’s Law of Independent Assortment states that parental alleles are
passed on independently to offspring, so inheriting an allele for one trait does
not impact inheriting another allele for a different trait (e.g., color and shape)

This occurs during meiosis when chromosomes are recombined (i.e., crossing-
over during prophase I) and rearranged (i.e., during metaphase I and II), leading
to random and independent assortment of alleles on different chromosomes. This
produces new combinations of alleles that increases genetic variation

Exception: genes that are located close to each other on the same chromosomes
are often inherited together, rather than being inherited independently; this is due to
genetic linkage, but we won't worry about that for now....

Mendel’s Law of Independent Assortment

Mendel’s Law of Independent Assortment

Mendel's Laws of Segregation/Separation + Independent Assortment

Parental gametes have randomly received the dominant or recessive alleles for color and shape

Recombination (crossing-over) has independently sorted the alleles for color and shape on the chromosomes, leading to unique combinations of alleles in all of the parental gametes

Karyotypes, Autosomes, & Sex Chromosomes

A karyotype is an image of a complete set of diploid
homologous chromosomes (46 total in humans)

Autosomes are any of the non-sex chromosomes with
the same structures (22 pairs of autosomes in humans)

Sex chromosomes (sometimes called allosomes)
make up the 23rd pair of chromosomes, and help
determine the biological sex of a given individual

Some traits are autosomal, and others are sex-linked,
controlled by genes/alleles on the sex chromosomes

Sex-linked Traits

Sex-linked traits are carried on the X
chromosome, not the Y chromosome

Males are usually more susceptible to
sex-linked conditions than females,
because they only inherit one X
chromosome

Recessive sex-linked traits are more
common (e.g., colorblindness, hemophilia)

Sex-linked Trait Practice

X^HX^H × X^hY

X^HX^h × X^hY

X^hX^h × X^HY

Pedigrees

Pedigrees are diagrams used to show the inheritance of a trait or health condition through multiple generations of a family

Circles represent females and squares represent males in pedigree charts

Filled-in symbols represent individuals with the trait/condition, blank symbols represent unaffected individuals; either can be heterozygous, depends on what background information you are given

Pedigrees

Pedigrees can be used to trace autosomal traits on any of the first 22 pairs of human chromosomes, or sex-linked traits located on the sex chromosome(s)

Parents are connected by horizontal lines, offspring of those parents by vertical lines and brackets; "marriage lines" connect offspring from different families who have interbred together

Multiple Choice

Based on this pedigree chart, is the autosomal trait being traced most likely dominant or recessive?

• Dominant

• Recessive

Multiple Choice

Based on this pedigree chart showing a recessive, autosomal trait, what is the genotype of the mother of the original set of parents?

• Hh

• hh

• HH

Multiple Choice

Based on this pedigree chart showing a recessive, sex-linked trait, what is the genotype of individual 2 in the first (parental) generation?

• X^hX^h

  (Homozygous Recessive)

• X^HX^H

  (Homozygous Dominant)

• X^HX^h

  (Heterozygous)

Non-Mendelian Inheritance

Many traits are not inherited via the simple Mendelian pattern with only dominant and recessive alleles for particular genes

Several non-Mendelian modes of inheritance also exist, including:

Incomplete Dominance

Codominance

Polygenic Traits

Epistasis

Multiple Alleles

Incomplete Dominance

In incomplete dominance, the dominant trait is not entirely dominant over the recessive trait, leading to a mixed offspring phenotype

Codominance

In codominance, both traits are equally dominant, so offspring that inherit both dominant alleles will exhibit both phenotypes

This is different from incomplete dominance because there are no intermediate mixed phenotypes; both dominant traits expressed

Polygenic Traits

Polygenic traits are traits in which the phenotype is controlled by at least two different genes acting together

Examples include skin color, eye color, hair color, height, etc.

Epistasis

Epistasis occurs when the expression of one gene depends on the expression of another gene (i.e., modifier gene)

For example, the presence of brown or black fur color may depend on the expression of a homozygous dominant or heterozygous modifier gene

Multiple Alleles

Some traits are even controlled by multiple alleles, or more than two versions of a single gene for a trait

Human blood types are controlled by multiple alleles, and also demonstrate codominance for A and B blood types over the recessive O blood type

Multiple Alleles

Multiple Choice

Which mode of non-Mendelian inheritance is represented by this diagram?

• Incomplete Dominance

• Codominance

• Multiple Alleles

• Epistasis

Multiple Choice

Which mode of non-Mendelian inheritance is represented by this diagram?

• Incomplete Dominance

• Codominance

• Multiple Alleles

• Epistasis

Multiple Select

Human blood types represent which of the following modes of non-Mendelian inheritance?

• Incomplete Dominance

• Codominance

• Multiple Alleles

• Epistasis

Multiple Choice

Which mode of non-Mendelian inheritance is exhibited by a population of birds in which feather color and wing length are both controlled by a set of superseding genes?

• Polygenic Traits

• Multiple Alleles

• Epistasis

• Codominance