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BIOLOGY - CLUTCH

CH.14 - MENDELIAN GENETICS

CONCEPT: MENDEL’S EXPERIMENT

Gregor Mendel designed an experiment to study inheritance in pea plants.

● Character – a feature that can be inherited, and shows variation between individuals

□ Trait – each version of a character

● The pea plant was a great model organism, an organism that is studied to elucidate widely applicable phenomena.

EXAMPLE:

● True breeding – an organism that, through self-fertilization, always produces offspring with the same trait of interest

● Hybridize – the cross-pollination of two true breeding organisms

EXAMPLE:

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● Parental Generation (P): true breeding parents are hybridized

● First Filial Generation (F1): the hybrid offspring of the parental generation, which are self-pollinated

● Second Filial Generation (F2): the offspring from self-fertilized F1 plants

□ Mendel noticed that the F2 traits for the same character were always found in the same ratio.

EXAMPLE:

● Allele – a version gene

● Traits are determined by two alleles, one from each parent, at the same gene locus.

□ Dominant Allele – determines organisms appearance

□ Recessive Allele – has no notable influence on organisms appearance

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● Phenotype – the observable traits of an organism

● Genotype – the genetic makeup of an organism (genotype phenotype)

□ Heterozygous – two different alleles for the same gene

□ Homozygous – two of the same allele for a gene

EXAMPLE:

● Monohybrid Cross – cross between monohybrids (heterozygous for one character), like those in F1

□ Punnett Square – a diagram representing the potential genetic outcomes of a cross

● Reciprocal Cross – a cross used to determine the influence of parental sex on inheritance

● Law of Segregation – two alleles for a given character separate during gamete formation, and go to different gametes

EXAMPLE:

Q q

Q

q

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PRACTICE: 1. Phenotype ratio: Genotype ratio:

2. Phenotype ratio:

Genotype ratio:

Q q

Q

Q

q

q

Q

Q

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PRACTICE: 3. Phenotype ratio:

Genotype ratio:

4. What did Mendel name the generations of plants in his experiments?

- - -

5. The allele Q is ______________________ to q, the _________________________ allele.

6. An organism that always produces offspring with the same trait of interest is said to be ___________________________

7. What process accounts for Mendel’s Law of Segregation?

q q

Q

q

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CONCEPT: INDEPENDENT ASSORTMENT

Mendel designed an experiment to examine the inheritance of two characters.

● Dihybrid Cross – cross of a dihybrid (heterozygous for two characters)

EXAMPLE:

● Independent Assortment – alleles for different characters sort independently of each other during gamete formation.

□ During meiosis, genes on different chromosomes will divide between daughter cells independently of each other.

● Test Cross – a method to determine the genotype of a phenotypically dominant plant

EXAMPLE:

YR Yr yR yr

YR

Yr

yR

yr

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CONCEPT: INDEPENDENT ASSORTMENT

Multiplication Rule: to determine the probability of two events both happening, multiply their individual probabilities.

Addition Rule: to determine the probability of either of two mutually exclusive events happening, add their probabilities.

EXAMPLE:

● Dihybrid cross probability:

EXAMPLE:

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PRACTICE: 1. What is the probability that the offspring of a dihybrid cross (YyRr x YyRr) will be yellow?

2. What is the probability that the offspring of a dihybrid cross (YyRr x YyRr) will be green and wrinkled?

3. What is the probability that the offspring of a dihybrid cross (YyRr x YyRr) will be yellow or round?

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CONCEPT: EXTENSIONS TO MENDEL

Alleles can show varying degrees of dominance:

● Complete dominance – the effect of one allele completely masks the other

● Incomplete dominance – heterozygous phenotype is distinct from homozygous phenotypes, and is often intermediate

EXAMPLE:

● Codominance – the effect of both alleles is seen in the phenotype

EXAMPLE:

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CONCEPT: EXTENSIONS TO MENDEL

Most genes have multiple alleles, like the ABO blood groups.

● Pleiotropy – when one gene influences multiple phenotypic traits.

EXAMPLE:

● Polygenic traits – when multiple genes influence one trait

● The environment can have an impact on phenotype.

● Multifactorial character – many factors influence phenotype, like genotype and environment

EXAMPLE:

● Epistasis – the expression of a gene at one locus alters the expression of a gene at another locus

EXAMPLE:

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PRACTICE: 1. A species of bird is known to have black beaks, yellow beaks, or spotted beaks, with patches of black and yellow. There

is a single gene locus that determines beak pigment, and the most likely mode of inheritance is _____________________?

a. Complete dominance

b. Incomplete dominance

c. Codominance

d. Partial dominance

2. A species of fish have tails that vary in size. It is found that genes at 2 loci, as well as diet, influence tail size. This trait is

an example of _________________________?

a. Pleiotropy

b. Polygenic inheritance

c. Epistasis

d. Multifactorial character

3. A ratio of 9:3:4 is produced during a heterozygote dihybrid cross when one gene pair interacts to control the expression of

the second gene pair. This interaction is called _________?

a. Dominance

b. Epigenetics

c. Epistasis

d. Recessive

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CONCEPT: GENETIC COUNSELING

Pedigree – a record of descent, mapping out inherited traits

● Mode of transmission – whether a trait is autosomal or sex-linked, and the type of dominance of the allele

EXAMPLE:

● Autosomal recessive pedigree:

□ Carriers – individuals who “carries” the recessive allele for a trait, but does not display the trait

EXAMPLE:

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● Autosomal dominant pedigree:

EXAMPLE:

● X-linked pedigree:

EXAMPLE:

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Genetic counseling relies on Mendel’s rules for inheritance and probability to help parents avoid genetic disorders.

● Amniocentesis – a type of fetal genetic testing that samples the amniotic fluid to test for markers of certain diseases

● Chorionic villus sampling (CVS) – a type of fetal genetic testing that samples tissue from the placenta, which is derived

from the fetus, and has the same genotype as the fetus

EXAMPLE:

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