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Date:
Class Period:
Meiosis and Mendelian Genetics
Vocabulary
_________________________________: the passing on of traits genetically from one
generation to another.
_________________________________: the scientific study of heredity
_________________________________: a specific characteristic that varies from one
individual to another
_________________________________: the chance that a particular event will happen
_________________________________: a diagram that shows the gene combinations that
might occur when two parents make offspring
_________________________________: having two identical alleles for a particular trait (aa or
AA)
_________________________________: having two different alleles for a particular trait (Aa)
_________________________________: the physical characteristics of an organism (such as
brown hair, freckles, or a big nose)
_________________________________: the genetic make-up of an organism (AA, Aa, or aa)
_________________________________: a gene that can cover up the phenotype of a recessive
allele
_________________________________: a gene whose trait can only be seen if there are no
dominant alleles present
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Meiosis Review
Recall that during meiosis, ________________________ _____________________________
pair
up in the middle of the cell to form ________________________ and cross over,
leading to new combinations of genes on each chromatid.
The four ___________________ from the tetrad each end up in a different
___________________________.
Check for Understanding
This diagram shows how chromosomes separate during meiosis.
Did these chromosomes cross over? How can you tell?
Gregor Mendel
Gregor Mendel was an Austrian monk who studied the traits of the pea plants
in his garden. He is often considered “The Father of Genetics.” His work was
very important to the understanding of genetics.
Mendelian Genetics
Mendelian genetics only works with ____________________ that are controlled by
one _________________. The presence of different ___________________ of that
_________________ determines the ____________________.
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Suppose this gene is the gene for a dimpled chin. A
dimpled chin is a trait that is only controlled by one gene,
meaning that there is one location (loci) on this
homologous pair of chromosomes that is for the dimpled
chin gene. There are no other genes anywhere, on any
chromosome, that control the dimpled chin
trait. Some traits, like height and skin color, are
controlled by many genes. For these traits, there are
several genes with different loci. The genes may even be
on different chromosomes.
Homozygous and heterozygous describe the
genotype of an organism.
An organism that is __________________________ for a
trait has two copies of the same allele. If two
dominant alleles are present, the organism is
__________________________ ______________________ for that
trait.
If two recessive alleles are present, the organism is
__________________________ ______________________ (PP or
aa).
An organism that is __________________________ for a trait has two different alleles, and
only the dominant allele is expressed in the phenotype (Bb).
Example
Dracula has a widow’s peak, which is a dominant trait. We know Dracula
must have at least one dominant allele for a widow’s peak (W). Dracula’s
genotype could be
homozygous dominant (WW) or heterozygous (Ww).
If Dracula had no widow’s peak, he would be homozygous
recessive (ww)
Check for Understanding
Having a dimple (or a cleft) in your chin is a dominant trait. The recessive
trait is having a smooth or straight chin.
What are the possible genotypes for Eminem’s chin? Use the letter D for the
dominant allele and d for the recessive allele.
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Recall: Chromosomes
_________ ________________________________ are the X and Y chromosomes
that determine an individual’s sex
__________________________________ are the rest of an individual’s
chromosomes
Humans have 23 pairs of chromosomes:
One pair of sex chromosomes
And 22 pairs of autosomes
Sex-Linked Traits
Alleles for ______ ___________________ _______________ can be located on either
the X or Y chromosome.
Most genes for sex-linked traits are located on the X chromosome (Xlinked), simply because the X chromosome has many more genes than the
Y chromosome.
Example: Colorblindness
Example: colorblindness is an X-linked, ___________________________ trait. Since
females have two X chromosomes, a female must have two alleles for
colorblindness to be colorblind (one on each X chromosome).
A female with only one allele for colorblindness is called a carrier, because she
will not express the colorblind phenotype, but she may pass the colorblind allele
to her offspring.
Since males only have one X chromosome, they will be colorblind if they have the
allele for colorblindness.
The genotype of a male affected with colorblindness would be _________________.
The genotype of a carrier female would be ______________________.
The genotype of female affected with colorblindness would be
________________________.
X-linked __________________________ traits are called recessive because the other, normal
_____________________ on the second X chromosome in females can __________________ the
trait.
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So, a female with two X chromosomes must have ________________
copies of the allele to express the X-linked recessive phenotype.
The Y chromosome has ________________genes than the X
chromosome, so it cannot mask the allele on the X chromosome.
The X and Y chromosomes are not homologous.
Check for Understanding
If a woman who is colorblind has a son with a man who is not colorblind, what is the
probability that the son will be colorblind?
If a colorblind woman has a daughter with a colorblind man, what is the probability
that the daughter will be colorblind?
Punnett Squares
A __________________ _______________ is a diagram that shows the
gene combinations that might occur when two parents make
offspring (children).
The _______________________ of one parent is listed across the top,
and the
______________________ of the other parent is listed down the left
side. The four boxes
inside represent the_________________________ ____________________________of the offspring.
Let’s fill in this Punnett square together.
Let’s say the trait we are looking at is for height in pea plants. T
represents the dominant tall allele, and t represents the
recessive short allele.
What are the genotypes of the parents in this Punnett square?
What is the probability of having tall offspring?
What is the probability of having short offspring?
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X-Linked Punnett Square
_________________________________ is an X-linked
genetic disease. It causes blood to clot
improperly, so people with the disease bleed
longer than people without the disease.
In this Punnett square, we are crossing a
____________________
________________________ with a male who has
hemophilia.
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