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Mendel's Experiments and Heredity > Laws of Inheritance
Laws of Inheritance
• Pairs of Unit Factors, or Genes
• Alleles Can Be Dominant or Recessive
• Equal Segregation of Alleles
• Independent Assortment
• Linked Genes Violate the Law of Independent Assortment
• Epistasis
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Mendel's Experiments and Heredity > Laws of Inheritance
Pairs of Unit Factors, or Genes
• A gene is a stretch of DNA that helps to control the development and function of
all organs and working systems in the body.
• Genes are passed from parent to offspring; the combination of these genes
affects all aspects of the human body, from eye and hair color to how well the liver
can process toxins.
• A human will inherit 23 chromosomes from its mother and 23 from its father;
together, these form 23 pairs of chromosomes that direct the inherited
characteristics of the individual.
• If the two copies of a gene inherited from each parent are the same, that
individual is said to be homozygous for the gene; if the two copies inherited from
Gene pairs enable genetic combinations
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each parent are different, that individual is said to be heterozygous for the gene.
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Mendel's Experiments and Heredity > Laws of Inheritance
Alleles Can Be Dominant or Recessive
• Dominant alleles are expressed exclusively in a heterozygote, while recessive
traits are expressed only if the organism is homozygous for the recessive allele.
• A single allele may be dominant over one allele, but recessive to another.
• Not all traits are controlled by simple dominance as a form of inheritance; more
complex forms of inheritance have been found to exist.
Recessive traits are only visible if an individual
inherits two copies of the recessive allele
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Mendel's Experiments and Heredity > Laws of Inheritance
Equal Segregation of Alleles
• Each gamete acquires one of the two alleles as chromosomes separate into
different gametes during meiosis.
• Heterozygotes, which posess one dominant and one recessive allele, can receive
each allele from either parent and will look identical to homozygous dominant
individuals; the Law of Segregation supports Mendel's observed 3:1 phenotypic
ratio.
• Mendel proposed the Law of Segregation after observing that pea plants with two
different traits produced offspring that all expressed the dominant trait, but the
following generation expressed the dominant and recessive traits in a 3:1 ratio.
The Law of Segregation states that alleles
segregate randomly into gametes
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Mendel's Experiments and Heredity > Laws of Inheritance
Independent Assortment
• Mendel's law of independent assortment states that genes do not influence each
other with regard to the sorting of alleles into gametes; every possible
combination of alleles for every gene is equally likely to occur.
• The calculation of any particular genotypic combination of more than one gene is,
therefore, the probability of the desired genotype at the first locus multiplied by the
probability of the desired genotype at the other loci.
• The forked line method can be used to calculate the chances of all possible
genotypic combinations from a cross, while the probability method can be used to
calculate the chance of any one particular genotype that might result from that
cross.
Independent assortment of 2 genes
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Mendel's Experiments and Heredity > Laws of Inheritance
Linked Genes Violate the Law of Independent Assortment
• Two genes close together on the same chromosome tend to be inherited together
and are said to be linked.
• Linked genes can be separated by recombination in which homologous
chromosomes exchange genetic information during meiosis; this results in
parental, or nonrecombinant genotypes, as well as a smaller proportion of
recombinant genotypes.
• Geneticists can use the amount of recombination between genes to estimate the
distance between them on a chromosome.
Unlinked genes assort independently
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Mendel's Experiments and Heredity > Laws of Inheritance
Epistasis
• In many cases, several genes may contribute to a particular phenotype; when the
actions of one gene masks the effects of another, this gene is said to be epistatic
to the second.
• Epistasis can occur when a recessive genotype masks the actions of another
gene, or when a dominant allele masks the effects of another gene.
• Epistasis can be reciprocal: either gene, when present in the dominant (or
recessive) form, expresses the same phenotype.
• Any single characteristic that results in a phenotypic ratio that totals 16 (such as
12:3:1, 9:3:4, or others) is typical of a two-gene interaction.
Epistasis in mouse coat color
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Appendix
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Mendel's Experiments and Heredity
Key terms
• chromosome a structure in the cell nucleus that contains DNA, histone protein, and other structural proteins
• dominant a relationship between alleles of a gene, in which one allele masks the expression (phenotype) of another allele at the
same locus
• epistasis the modification of the expression of a gene by another unrelated one
• gene a unit of heredity; the functional units of chromosomes that determine specific characteristics by coding for specific
proteins
• genetics the branch of biology that deals with the transmission and variation of inherited characteristics, in particular
chromosomes and DNA
• independent assortment separate genes for separate traits are passed independently of one another from parents to offspring
• law of segregation a diploid individual possesses a pair of alleles for any particular trait and each parent passes one of these
randomly to its offspring
• linkage the property of genes of being inherited together
• recessive able to be covered up by a dominant trait
• recombination the formation of genetic combinations in offspring that are not present in the parents
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Mendel's Experiments and Heredity
Gene pairs enable genetic combinations
A child will inherit half of its genes (one of each of its 23 pairs) from its mother and the other half from its father.
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Mendel's Experiments and Heredity
Recessive traits are only visible if an individual inherits two copies of the recessive allele
The child in the photo expresses albinism, a recessive trait.
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Mendel's Experiments and Heredity
The Law of Segregation states that alleles segregate randomly into gametes
When gametes are formed, each allele of one parent segregates randomly into the gametes, such that half of the parent's gametes carry each allele.
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Mendel's Experiments and Heredity
Independent assortment of 2 genes
This dihybrid cross of pea plants involves the genes for seed color and texture.
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Mendel's Experiments and Heredity
Independent assortment of 3 genes
The forked-line method can be used to analyze a trihybrid cross.Here, the probability for color in the F2 generation occupies the top row (3 yellow:1
green).The probability for shape occupies the second row (3 round:1 wrinked), and the probability for height occupies the third row (3 tall:1 dwarf).The
probability for each possible combination of traits is calculated by multiplying the probability for each individual trait.Thus, the probability of F2 offspring
having yellow, round, and tall traits is 3 × 3 × 3, or 27.
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Mendel's Experiments and Heredity
Linked genes can be separated by recombination
The process of crossover, or recombination, occurs when two homologous chromosomes align during meiosis and exchange a segment of genetic
material.Here, the alleles for gene C were exchanged.The result is two recombinant and two non-recombinant chromosomes.
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Mendel's Experiments and Heredity
Unlinked genes assort independently
This figure shows all possible combinations of offspring resulting from a dihybrid cross of pea plants that are heterozygous for the tall/dwarf and
inflated/constricted alleles.
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Mendel's Experiments and Heredity
Epistasis in mouse coat color
In mice, the mottled agouti coat color (A) is dominant to a solid coloration, such as black or gray.A gene at a separate locus (C) is responsible for
pigment production.The recessive c allele does not produce pigmentnand a mouse with the homozygous recessive cc genotype is albino regardless of
the allele present at the A locus.Thus, the C gene is epistatic to the A gene.
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Mendel's Experiments and Heredity
A horse has 64 chromosomes. How many does it inherit from
each parent?
A) 32
B) 64
C) 16
D) 8
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Mendel's Experiments and Heredity
A horse has 64 chromosomes. How many does it inherit from
each parent?
A) 32
B) 64
C) 16
D) 8
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Mendel's Experiments and Heredity
Which of the following traits is NOT controlled by genetics?
A) Hair color
B) A poison oak rash
C) Shoe size
D) Colon cancer
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Mendel's Experiments and Heredity
Which of the following traits is NOT controlled by genetics?
A) Hair color
B) A poison oak rash
C) Shoe size
D) Colon cancer
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Mendel's Experiments and Heredity
If an individual expresses a phenotype associated with a
dominant gene, does he or she have a recessive allele for that
gene?
A) You cannot tell unless you know what the parent's phenotypes are.
B) You cannot tell unless you know the genotype of that person.
C) You cannot tell unless you know how old the offspring are.
D) You cannot tell unless you know how many offspring are produced.
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Mendel's Experiments and Heredity
If an individual expresses a phenotype associated with a
dominant gene, does he or she have a recessive allele for that
gene?
A) You cannot tell unless you know what the parent's phenotypes are.
B) You cannot tell unless you know the genotype of that person.
C) You cannot tell unless you know how old the offspring are.
D) You cannot tell unless you know how many offspring are produced.
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Online Courses Built by Professors." CC BY 3.0 http://www.saylor.org/courses/bio102/
Mendel's Experiments and Heredity
A plant with red flowers (Rr) is crossed with another plant with red
flowers (Rr). What is the probability that an offspring from this
cross will have white flowers (rr)?
A) 1/2
B) 3/4
C) 1/4
D) 0
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Mendel's Experiments and Heredity
A plant with red flowers (Rr) is crossed with another plant with red
flowers (Rr). What is the probability that an offspring from this
cross will have white flowers (rr)?
A) 1/2
B) 3/4
C) 1/4
D) 0
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Mendel's Experiments and Heredity
For human blood type, the A and B alleles are codominant, while
the O allele is recessive. If an AB mother had children with a B
blood type father (genotype BO), what proportion of the children
would express the B protein alone?
A) 1/4
B) 1/2
C) 3/4
D) 0/4
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Mendel's Experiments and Heredity
For human blood type, the A and B alleles are codominant, while
the O allele is recessive. If an AB mother had children with a B
blood type father (genotype BO), what proportion of the children
would express the B protein alone?
A) 1/4
B) 1/2
C) 3/4
D) 0/4
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Mendel's Experiments and Heredity
If the genotype of one animal is AaBb, and the genotype of the
other animal is Aabb, what percentage of their offspring will be
AaBb?
A) 9/16
B) 1/4
C) 1/2
D) 3/16
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Mendel's Experiments and Heredity
If the genotype of one animal is AaBb, and the genotype of the
other animal is Aabb, what percentage of their offspring will be
AaBb?
A) 9/16
B) 1/4
C) 1/2
D) 3/16
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Mendel's Experiments and Heredity
Two genes, A and B, are linked. Consider that one parent has the
genotype AaBb and the other has the genotype aabb. Which of
the following genotypes would NOT be considered a recombinant
in the offspring?
A) AaBb
B) It is impossible to tell.
C) aaBb
D) Aabb
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Mendel's Experiments and Heredity
Two genes, A and B, are linked. Consider that one parent has the
genotype AaBb and the other has the genotype aabb. Which of
the following genotypes would NOT be considered a recombinant
in the offspring?
A) AaBb
B) It is impossible to tell.
C) aaBb
D) Aabb
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Mendel's Experiments and Heredity
Two individuals, both with the genotype AaBb, produce a number
of offspring. The dominant "A" allele masks the effect of the "B"
genotype. What phenotypic ratio do you expect to see in the
offspring?
A) 12:3:1
B) 9:3:3:1
C) 9:3:4
D) 15:1
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Mendel's Experiments and Heredity
Two individuals, both with the genotype AaBb, produce a number
of offspring. The dominant "A" allele masks the effect of the "B"
genotype. What phenotypic ratio do you expect to see in the
offspring?
A) 12:3:1
B) 9:3:3:1
C) 9:3:4
D) 15:1
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Mendel's Experiments and Heredity
Attribution
• Wikibooks. "Human Physiology/Genetics and inheritance." CC BY-SA 3.0
http://en.wikibooks.org/wiki/Human_Physiology/Genetics_and_inheritance
• Connexions. "Laws of Inheritance." CC BY 3.0 http://cnx.org/content/m44479/latest/?collection=col11448/latest
• Wiktionary. "genetics." CC BY-SA 3.0 http://en.wiktionary.org/wiki/genetics
• Wiktionary. "chromosome." CC BY-SA 3.0 http://en.wiktionary.org/wiki/chromosome
• Wiktionary. "gene." CC BY-SA 3.0 http://en.wiktionary.org/wiki/gene
• Connexions. "Laws of Inheritance." CC BY 3.0 http://cnx.org/content/m44479/latest/?collection=col11448/latest
• Wikipedia. "Dominance (genetics)." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Dominance_(genetics)
• Wikipedia. "Recessive." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Recessive
• Wiktionary. "recessive." CC BY-SA 3.0 http://en.wiktionary.org/wiki/recessive
• Wikipedia. "dominant." CC BY-SA 3.0 http://en.wikipedia.org/wiki/dominant
• Wikipedia. "law of segregation." CC BY-SA 3.0 http://en.wikipedia.org/wiki/law%20of%20segregation
• Wikibooks. "Structural Biochemistry/Foundation of Genetics." CC BY-SA 3.0
http://en.wikibooks.org/wiki/Structural_Biochemistry/Foundation_of_Genetics
• Connexions. "Laws of Inheritance." CC BY 3.0 http://cnx.org/content/m44479/latest/?collection=col11448/latest
• Wikipedia. "independent assortment." CC BY-SA 3.0 http://en.wikipedia.org/wiki/independent%20assortment
• Connexions. "Laws of Inheritance." CC BY 3.0 http://cnx.org/content/m44479/latest/?collection=col11448/latest
• Connexions. "Laws of Inheritance." CC BY 3.0 http://cnx.org/content/m44479/latest/?collection=col11448/latest
• Wiktionary. "recombination." CC BY-SA 3.0 http://en.wiktionary.org/wiki/recombination
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Mendel's Experiments and Heredity
• Wiktionary. "linkage." CC BY-SA 3.0 http://en.wiktionary.org/wiki/linkage
• Connexions. "Laws of Inheritance." CC BY 3.0 http://cnx.org/content/m44479/latest/?collection=col11448/latest
• Wiktionary. "epistasis." CC BY-SA 3.0 http://en.wiktionary.org/wiki/epistasis
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