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Genetics. The study of heredity. Genetics. Genetics is the scientific study of heredity - how traits are passed from generation to generation. The characteristics that are inherited are called traits . Genes. Humans have 23 homologous pairs of chromosomes.

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Genetics

Genetics

The study of heredity


Genetics1
Genetics

  • Genetics is the scientific study of heredity - how traits are passed from generation to generation.

  • The characteristics that are inherited are called traits.


Genes
Genes

  • Humans have 23 homologous pairs of chromosomes.

  • On each chromosomes, there are sections called genes that code for traits.



Alleles
Alleles

  • An allele is a distinct form of a gene.

  • Every person has 2 alleles for a gene  1 from the father and 1 from the mother


Alleles1
Alleles

  • Letters of the alphabet are used to represent an allele of interest.

  • Every person has two copies of an allele, so they will have two letters.

T

P

Y

A

R


Alleles2
Alleles

  • Dominantalleles are symbolized with a capital letter. Dominant alleles will mask a recessive alleles in cases of simple dominance/recessiveness.

  • Recessive alleles are symbolized with lower case letters.

A

a


Homozygous alleles
Homozygous Alleles

  • If an organism has two like copies of an allele, it is homozygous (homo = same).

  • If the two alleles are dominant, the organism is homozygous dominant.

  • If the two alleles are recessive, the organism is homozygous recessive.

AA

aa


Heterozygous alleles
Heterozygous Alleles

  • If an organism has two different copies of an allele, it is heterozygous (hetero = different).

Aa


Genotype and phenotype
Genotype and Phenotype

  • The letters an organism has represent the organism’s genotype - what alleles the organism has.

  • As a result of the alleles present, a trait is expressed. The phenotype is the expressed trait.


Genotype and phenotype1
Genotype and Phenotype

  • Example: In a plant species, there are two alleles for flower color: R and r.

  • R is dominant, and codes for red flowers

  • r is recessive and codes for white flowers


Genotype and phenotype2
Genotype and Phenotype

  • The genotype is the combination of alleles: either RR, Rr, or rr.

  • The phenotype is what is expressed: either red or white flowers.


Genotype and phenotype3
Genotype and Phenotype

  • RR -

    • Homozygous dominant

    • Red flowers

  • Rr -

    • Heterozygous dominant

    • Red flowers

  • rr -

    • Homozygous recessive

    • White flowers

In cases of simple dominance, an organism must have two copies of the recessive alleles to express the recessive trait.


Purebreds and hybrids
Purebreds and Hybrids

  • Purebred - an organism that receives the same genetic traits from both of its parents

  • Hybrid - an organism that receives different forms of a genetic trait (different alleles) from each parent


Mendel s laws

Mendel’s Laws

Contributions of Gregor Mendel


Law of dominance
Law of Dominance

  • The dominant alleles is expressed and may mask a recessive allele. The recessive form of a trait is only shown in a homozygous recessive organism.

  • Ex. R is allele for round, r is allele for square.

    • RR - round

    • Rr - round

    • rr - square


Law of segregation
Law of Segregation

  • Gene pairs separate when gametes are formed.

Parent:

Dd

Parent:

dd

D

d

d

d

Gametes

Gametes


Law of independent assortment
Law of Independent Assortment

  • Genes segregate randomly and independently. This means that if there are 2 or more traits, every combination of those traits is possible.

AbC

Abc

abC

abc

AabbCc


Probability and punnett squares

Probability and Punnett Squares

Predicting the genotypes and phenotypes of offspring


Probability
Probability

  • Probability - the likelihood that a particular event will occur (what are the odds?)

  • What is the probability that a single coin flip comes up heads?

    • 50% or 1/2


Probability1
Probability

  • True or False? The past outcomes of coin flips greatly affects the outcomes of future coin flips.

  • False.

    • There’s still a 50% chance of heads and 50% chance of tails!


Probability2
Probability

  • The way in which alleles separate is random, like a coin flip. (Mendel’s Law of Segregation)

  • From a mother who is heterozygous for an allele, there is a 50% chance she passes on the dominant allele and a 50% chance she passes on the recessive allele.


Punnett squares
Punnett Squares

  • Punnett squares show probabilities for genotypes and phenotypes of offspring of two parent organisms.

  • Example:

    • In Mendel’s pea plants, the plants had either purple (P) or white (p) flowers.


Punnett squares1
Punnett Squares

  • Step 1. Make the grid.

    • If there is 1 trait, it is a 2x2 grid.

    • If there are 2 traits, it is a 4x4 grid.

  • Because we are only looking at 1 trait (flower color), a 2x2 grid is needed.


Punnett squares2
Punnett Squares

Pp

  • Step 2: Determine the parents’ genotypes and possible gametes.

  • Example: a heterozygous pea plant and a homozygous dominant pea plant.

P

p

P

PP

P


Punnett squares3
Punnett Squares

Pp

  • Step 3: Fill in the squares by combining what is on top of the column and to the left of the row.

P

p

P

PP

Pp

PP

PP

Pp

P


Punnett squares4
Punnett Squares

Pp

  • Step 4: Use the Punnett square to determine probabilities and ratios.

P

p

P

PP

Pp

PP

PP

Pp

P


Punnett squares5
Punnett Squares

  • What is the probability of an offspring plant having purple flowers?

    • 100%

  • What is the probability of an offpsring plant being heterozygous?

    • 2/4 = 1/2 = 50%

PP

Pp

PP

Pp


Punnett squares6
Punnett Squares

  • If there are 2 traits, the Punnett square will be a 4x4 grid.

  • Example: Cross a pea plant that is heterozygous for both flower color and seed shape with a plant that has white flowers and is heterozygous for seed shape

P - purple; p - white

R - round, r - wrinkled


Punnett squares7
Punnett Squares

  • Cross a pea plant that is heterozygous for both flower color and seed shape with a plant that has white flowers and is heterozygous for seed shape

PpRr

ppRr

PR, Pr, pR, pr

pR, pr, pR, pr


Punnett squares8
Punnett Squares

ppRr

pR

pR

pr

pr

PR

Pr

PpRr

pR

pr


Punnett squares9
Punnett Squares

ppRr

pR

pR

pr

pr

PR

Pr

PpRr

pR

pr


Punnett squares10
Punnett Squares

ppRr

pR

pR

pr

pr

PR

Pr

PpRr

pR

pr


Punnett squares11
Punnett Squares

pR

pR

pr

pr

  • What is the probability of an offspring having white flowers and wrinkled seeds?

PR

2 / 16 = 1 / 8

or

12.5%

Pr

pR

pr


Punnett squares12
Punnett Squares

pR

pR

pr

pr

  • What is the probability of an offspring having purple flowers and round seeds?

PR

6 / 16 = 3 / 8

or

37.5%

Pr

pR

pr


Punnett squares13
Punnett Squares

  • Write the probable genotypic ratio.

  • 2 PpRR : 4 PpRr : 2 Pprr : 2 ppRR : 4 ppRr : 2 pprr

  • 1 PpRR : 2 PpRr : 1 Pprr : 1 ppRR : 2 ppRr : 1 pprr


Intermediate inheritance

Intermediate Inheritance

Beyond Simple Dominance


Intermediate inheritance1
Intermediate Inheritance

  • There are 3 types of intermediate inheritance, genetic patterns that don’t follow the simple dominant-recessive rules.

    • Incomplete dominance

    • Codominance

    • Multiple alleles


Incomplete dominance
Incomplete Dominance

  • Incomplete dominance - neither allele is completely dominant over the other

  • The heterozygous form is a “blended” form of the two alleles.


Incomplete dominance1
Incomplete Dominance

  • Example: In snapdragon flowers, there is an allele that codes for red (r), and allele that codes for white (w).

  • rr - red

  • ww - white

  • rw - pink


Incomplete dominance2
Incomplete Dominance

  • Ex. Cross a red and a pink snapdragon.

r

w

r

r


Incomplete dominance3
Incomplete Dominance

  • Sometimes two like capital letters are used, but one gets a prime sign (‘).

  • Ex: Human hair

    • Curly hair HH

    • Straight hair H’H’

    • Wavy hair HH’


Codominance
Codominance

  • Codominance - both alleles are dominant and get expressed equally

  • In the heterozygous has some of each phenotype, but they are not blended.


Codominance1
Codominance

  • Example - in a type of cattle, red hair (R) and white hair (W) are codominant.

  • RR - red

  • WW - white

  • RW - roan

    • Some red, some white, but not pink!


Codominance2
Codominance

  • Ex. Cross a red parent and a white parent.

R

R

W

W


Multiple alleles
Multiple Alleles

  • Multiple alleles - there are more than 2 alleles for a trait.

  • Ex. Fur color - gray, black, striped

  • Ex. Human blood types


Sex linkage

Sex Linkage

Sex-linked, sex-limited,

and sex-influenced traits


Human chromosomes
Human Chromosomes

  • Humans have 23 homologous pairs of chromosomes, for a total of 46.

  • 22 pairs are called autosomes , which are all of the non-sex chromosomes

  • The 23rd pair is the sex chromosomes - X and Y.


Sex chromosomes
Sex Chromosomes

  • X and Y

    • Females - XX

      • All eggs have an X

    • Males - XY

      • Sperm have either an X or Y


Sex linked traits
Sex-Linked Traits

  • Traits controlled by genes on the sex chromosomes are sex-linked traits.

  • Examples of sex-linked traits: hemophilia, color blindness, male pattern baldness

  • Most are “attached” to the X chromosome.

    • Therefore, females have 2 copies of these alleles and males only have one


Example hemophilia
Example - Hemophilia

  • Hemophilia - a blood clotting disorder

  • Hemophilia is X-linked.

  • XH = normal

  • Xh = hemophilia

  • Y is still just a Y


Example hemophilia1
Example - Hemophilia

  • Females could be:

    • XHXH - don’t have hemophilia, not a carrier

    • XHXh - don’t have hemophilia, is a carrier

    • XhXh - have hemophilia


Example hemophilia2
Example - Hemophilia

  • Males can be:

    • XHY - does not have hemophilia

    • XhY - has hemophilia

      • Males cannot be carriers - they either have it or they don’t!


Example hemophilia3
Example - Hemophilia

  • Draw a Punnett square for cross between a carrier female and an unaffected male.

  • Female: XHXh Male: XHY

XH

Y

XH

Xh


Example hemophilia4
Example - Hemophilia

  • What is the percent chance that a child of theirs will have the disorder?

    • 25%

XH

Y

XH

Xh


Example hemophilia5
Example - Hemophilia

  • What is the percent chance that a child of theirs will have the disorder?

    • 25%

XH

Y

XH

Xh


Example hemophilia6
Example - Hemophilia

  • What is the percent chance that a a son would have the disorder?

    • 50%

XH

Y

XH

Xh


Example hemophilia7
Example - Hemophilia

  • What is the percent chance that a daughter would be a carrier?

    • 50%

XH

Y

XH

Xh


Example colorblindness
Example - Colorblindness

  • Color blindness is also X-linked.

  • X = normal

  • Xc = colorblind


Example colorblindness1
Example - Colorblindness

  • Cross a colorblind male and a carrier female.

Xc

Y

X

Xc


Sex limited traits
Sex-limited traits

  • Sex-limited traits are only expressed in the presence of sex hormones, or are only observed in one sex or the other.

    • Ex. Beard growth


Sex influenced traits
Sex-influenced traits

  • Sex-influenced traits are expressed in both sexes, but they are expressed differently.

    • Ex. Baldness is dominant in men, recessive in women



Pedigrees1
Pedigrees

  • Males

  • Females

  • Affected - shaded

  • Unaffected - not shaded

  • Carrier - half shaded


Pedigrees2
Pedigrees

  • A pedigree is a diagram showing family history and tracing a genetic trait.


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