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Genetics. Study of inheritance, the stability and variance of inheritance patterns Gregor Mendel, “The Father of Genetics” Worked with the garden pea plant using cross pollination What would happen if we all had the exact same DNA sequences? What if we had extremely divergent DNA sequences?

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  • Study of inheritance, the stability and variance of inheritance patterns

  • Gregor Mendel, “The Father of Genetics”

  • Worked with the garden pea plant using cross pollination

  • What would happen if we all had the exact same DNA sequences? What if we had extremely divergent DNA sequences?

  • What determines which genes we inherit from our mother and father?

Genetic terms
Genetic Terms

  • Gene-basic unit of inheritance, arranged in a linear sequences on a chromosomes

  • Allele-alternative forms of a gene; alleles contain genetic information that is expressed as traits.

  • Examples of traits are flower color, seed shape, plant height etc.

  • Each trait can have a different version (allele); flower color can be purple or white, we use one letter for the trait flower color, but change whether it is upper or lower case

Genetic terms1
Genetic Terms

  • Dominant- 1 copy of allele results in expression of trait, always use a capital letter to show it is dominant, ex. A

  • Recessive-requires 2 copies of same allele to get expression of trait, always use lower case letter to show it is recessive, ex. a

  • If purple is dominant to white flowers, how would you write your alleles?

Genetic terms2
Genetic Terms

  • Homozygous Dominant-2 same copies of dominant allele, get expression of dominant trait, AA

  • Heterozygous Dominant-1 copy of dominant allele, 1 copy of recessive, still get dominant trait expression, Aa

  • Homozygous recessive-2 copies of recessive allele, get expression of recessive trait, aa

Genetic terms3
Genetic Terms

  • Genotype-genes of an organism

  • Phenotype-physical appearance of the organism

    True or False: The phenotype of an organism is dictated by the genotype of that organism

  • True breed lineage vs. hybrid offspring

  • P, F1, F2 Generation

Genetic terms4
Genetic Terms

  • Monohybrid cross-a cross that only predicts the genotype and phenotype for one trait, ex. plant flower color

  • Dihybrid cross-a cross that predicts the genotype and phenotype of 2 traits, ex. plant flower color and height

Predicting gametes
Predicting Gametes

If an organism has the genotype BB, how many different gametes can it make? 1: B

When predicting gametes, keep in mind that each gamete must have 1 complete set of instructions


2: C, c 1: DE 4: GH, Gh, gH, gh


2 1 4

Mendel s early genetic experiments monohybrid cross
Mendel’s Early Genetic Experiments: Monohybrid Cross

Crossed a true breed purple flower plant with a true breed white flower plant

P generation PP x pp P-purple



F1 generation, all purple heterozygous dominant

Crossed the F1 generation with each other

Pp x Pp

F2 generation PP, Pp, Pp, pp

Genotypic Ratio 1 PP: 2 Pp :1 pp

Phenotypic Ratio 3 purple: 1 white

Mendel s law of segregation
Mendel’s Law of Segregation

  • Mendel continued his monohybrid crosses (1000s) for 7 different traits

  • Average ratio for all traits studied was always about 3:1

  • The Law of Segregation:

    • Each individual has 2 of every gene (alleles) for each trait

    • These genes (alleles) will separate from each other during meiosis into different gametes.

    • Fertilization gives each new individual 2 alleles for each trait














Genetics problem
Genetics Problem

  • In parrots, alleles for blue feathers are dominant to alleles for yellow.

    Cross a heterozygous dominant blue feathered parrot with a yellow feathered parrot

Monohybrid genetics problem

Monohybrid Genetics Problem

B b



Genotypic ratio 1 Bb : 1 bb

Phenotypic ratio 1 Blue : 1 yellow

Genetics problem1
Genetics Problem

  • Rhinoceroses can be born without a horn, the recessive condition.

  • Cross 2 heterozygous dominant rhinoceroses. Can they produce a baby rhino without a horn?

Monohybrid cross

Monohybrid Cross

H-horn present

h-no horn





Genotypic Ratio 1 HH: 2 Hh : 1 hh

Phenotypic Ratio 3 with horns : 1 no horn

Mendel s early genetic experiments dihybrid cross
Mendel’s Early Genetic Experiments: Dihybrid Cross

  • Mendel noticed that all purple flower plants were tall and all white flower plants were short

  • Could you ever see a purple short plant or a white tall plant?

  • Crossed a homozygous dominant tall purple flower plant with a homozygous recessive short white flower plant

Mendel s early genetic experiments dihybrid cross1

Mendel’s Early Genetic Experiments: Dihybrid Cross

P generation AABB x aabb

F1 AaBb

F2 AaBb x AaBb

4 by 4 punnett square

Phenotypic ratio 9 purple, tall: 3 purple, short: 3 white, tall: 1 white, short

Mendel s early genetic experiments principle of independent assortment
Mendel’s Early Genetic Experiments: Principle of Independent Assortment

  • The results from the many dihybrid crosses allowed him to develop the

  • Principle of Independent Assortment-gene pairs (traits) are independent of each other and are sorted into different gametes

  • Exception: Linked genes; they always sort together into same gamete


A Independent Assortment




Diploid (2N)









Haploid (1N)

Genetics problem dihybrid cross

Genetics Problem: Independent AssortmentDihybrid Cross

In panthers, black fur is dominant to yellow fur.

A recessive gene results in the absence of claws.

Predict the offspring of a cross between a heterozygous black panther with no claws and a yellow panther that is heterozygous for claws

Genetics problem dihybrid cross1

Genetics Problem: Dihybrid Cross Independent Assortment

B-black fur b-yellow fur

C-claws c-no claws

bC bc



Genotypic Ratio 1:1:1:1

Phenotypic Ratio 1:1:1:1

Punnett square and probabilities
Punnett Independent Assortment Square and Probabilities

  • What is the probability from the following cross that any offspring will have unattached earlobes or attached earlobes?

  • The probability of inheriting a specific allele is like flipping a coin and occurs every time parents have an offspring

Mendel s early genetic experiments test cross
Mendel’s Early Genetic Experiments: Test Cross Independent Assortment

  • If you have a purple flower, what are the possible genotypes?

    PP or Pp

  • How do you decide? Do a Test Cross using a white flower plant

    PP x pp = all offspring are Pp (purple)

    Pp x pp = ½ offspring are Pp (purple)

    ½ offspring are pp (white)

Human genetics
Human Genetics Independent Assortment

  • The study of inheritance and prediction of genes in humans

  • Very difficult, many genes involved

  • Small sample size, few offspring

  • Mate by chance, live in diverse environments

  • Long life span makes it difficult to track genes in different generations

Human genetics understanding a pedigree
Human Genetics: Understanding a Pedigree Independent Assortment

  • Pedigree-a chart of genetic connections between individuals; family tree that tracks genes/diseases

  • Single genes can be followed by constructing pedigrees

  • Square=male circle=female

  • Shaded =affected non-shaded=not affected

  • A line between a circle + square=mating

  • Lines from the mating=offspring

Human inheritances patterns
Human Inheritances Patterns Independent Assortment

  • Autosomal Recessive Inheritance-gene is located on autosome; 2 copies of gene required for expression of disease/trait; 1 copy=carrier, not affected

  • Albinism, cystic fibrosis, sickle cell anemia, phenylketonuria, methemoglobinemia, Niemann-Pick disease

Autosomal recessive inheritance

F-normal, no cystic fibrosis f-cystic fibrosis Independent Assortment

Autosomal Recessive Inheritance

What are the chances of offspring from 2 heterozygote parents for the cystic fibrosis gene having the disease?

Parents are carriers

F f



25% (1 out of 4) will have cystic fibrosis

What are the genotypic/phenotypic ratios?

Human inheritances patterns1
Human Inheritances Patterns Independent Assortment

  • Autosomal dominant inheritance- gene is located on autosome;1 copy of gene results in expression of disease/trait

  • Ex. Achondroplasia, Huntington’s, OsteogenesisImperfecta, polydactyly, progeria, sperocytosis

Autosomal dominant inheritance

Autosomal Dominant Inheritance Independent Assortment

P-polydactyly (extra fingers/toes) p-normal

What are the chances of offspring from a cross of 2 heterozygous parents for polydactyly also having the condition?

P p



75% (3 out of 4) chance of having polydactyly

What is the genotypic/phenotypic ratios?

Variations to mendelian inheritance patterns multiple alleles
Variations to Mendelian Inheritance Patterns: Multiple Alleles

  • Codominance

  • 2 alleles are not dominant to each other, and if both are present both are expressed

  • Ex. Blood Groups- ABO blood typing

  • A dominant to O, but not to B

  • B dominant to O, but not to A

  • O recessive

Codominance blood typing
Codominance: Blood Typing Alleles

  • There are 6 genotypes that express 4 different blood phenotypes

    Phenotype Genotype

  • Type A AA, AO

  • Type B BB, BO

  • Type AB AB

  • Type O OO

Codominance blood typing1
Codominance: Blood Typing Alleles

  • What does a blood typing do? Why is blood called A, B, AB, or O?

  • Based on the different sugars found on red blood cells ex. Type A blood has A sugars on RBCs

  • What type of sugars does AB or O have?

  • Why do we need to type blood?

Codominance blood typing problems
Codominance: Blood Typing Problems Alleles

  • Cross a person with type O blood with one that has type AB blood

  • Give phenotypic and genotypic ratios

Codominance blood typing problems1

Codominance: Blood Typing Problems Alleles




Genotypic Ratio 1 AO : 1 BO

Phenotypic Ratio 1 Type A : 1 Type B

Codominance blood typing problems2

Codominance: Blood Typing Problems Alleles

Type B blood male and a Type O blood female never produce a Type O blood child.

Is this possible? Why or why not?

Codominance blood typing problems3

Codominance: Blood Typing Problems Alleles

Type B blood can be BB or BO*




Genotypic Ratio All are BO

Phenotypic Ratio All are Type B

*this genotype would produce a Type O offspring

Blood type problem

Blood Type Problem Alleles

If father’s genotype is BO, it would be possible to get an O blood type child




Genotypic ratio: 1 BO: 1 OO

Phenotypic ratio: 1 type B: 1 type O blood

Variations to mendelian inheritance patterns
Variations to AllelesMendelian Inheritance Patterns

  • Incomplete Dominance-one allele isn’t complete dominant to the other; heterozygotes are intermediate in phenotype

  • Snap dragon flower color: R-red, r-white

  • RR x rr = Rr all offspring are pink

  • What happens when 2 pink flower plants are crossed? Give phenotypic and genotypic ratios



Incomplete dominance

Incomplete Dominance Alleles

R-red r-white Rr-pink

R r



Pleiotropy Alleles

  • Expression of alleles (for 1 trait) has positive or negative effects on other traits Ex. Sickle cell mutation

  • Affects the protein hemoglobin which carries O2

  • Insufficient O2 will cause RBCs to sickle and eventually burst anemia

  • Secondary effectsheart/lung damage, kidney/heart failure, skull deformation, mental impairment

Pleiotropy marfan syndrome
Pleiotropy: Marfan Syndrome Alleles

  • Single gene mutation affects 2 or more distinct and unrelated traits

  • mutation of fibrillin gene

Incomplete penetrance
Incomplete Penetrance Alleles

  • Polydactyly  extra fingers/toes

  • Autosomal dominant disorder which exhibits incomplete penetrance

  • A dominant allele sometimes does not determine the phenotype

  • Some who inherit polydactyly allele are phenotypically normal

Pleiotropy sickle cell anemia
Pleiotropy: Sickle Cell Anemia Alleles

  • Homozygous for condition die in early 40s, no cure, extremely debilitating

  • Severe anemia, poor circulation, physical weakness, impaired mental function, spleen damage

  • Why is this mutation maintained?

  • Protection/resistance against malaria

    S-no sickle cell s-sickle cell (recessive)

  • SS-no sickle cell, no resistance

  • Ss-no sickle cell, resistance

  • ss-sickle cell, resistance

Heterozygote advantage

Sickle cell anemia genetics problem

Sickle Cell Anemia Genetics Problem Alleles

Cross 2 heterozygous dominant parents together.

How many children would have sickle cell?

How many children would not?

How many children are resistant to malaria? How many are not?

Sickle cell anemia genetics problem1

Sickle Cell Anemia Genetics Problem Alleles

Parents are Ss-do not have sickle cell

S s



3 children do not have sickle cell, 1 does

3 children protected, 1 is not

Epistasis Alleles

  • When one gene pair masks/prevents another gene pair’s expression

    Ex. Labrador fur color

  • B-black fur b-brown fur

  • E-melanin deposited e-no melanin

  • The recessive genotype of “ee” will cause no melanin deposition, thus the resulting fur coat will be yellow, even when “B or b” alleles are present

Continuous variation in traits
Continuous Variation in Traits Alleles

  • Multiple genes are responsible for the phenotype of an organism  polygenic inheritance

  • Skin and eye color, height

  • A great deal of variation exists resembling a bell shaped curve

  • Look at human height; a few genes regulate height, but there exists a normal amount of variation

  • What factors contribute to height?

Environmental effects on phenotype multifactorial traits
Environmental Effects on Phenotype: Multifactorial Traits Alleles

  • Fur color on Siamese cats or Himalayan rabbits-heat sensitive enzyme that produces melanin

  • Flower color on Hydrangea Plant-influenced by acidity of soil

  • Height of Yarrow plant cuttings-varies depending on elevation planted

Continuous variation in traits1
Continuous Variation in Traits Alleles

  • Genetics allows us to predict genotypes of organisms, but there are many external and internal factors that influence the actual phenotype of those organisms  cleft lip/palate, clubfoot, hypertension, diabetes, schizophrenia, allergies, cancers

  • An individual’s phenotype is the outcome of the complex interaction among all its genes and environment in which it lives

  • G x E interaction

Sex determination in humans
Sex Determination in Humans Alleles

  • Determined by the 23rd pair on chromosomes

  • XX-female XY-male

  • A female only makes eggs that carry the X chromosome

  • A male makes sperm that contain either the X or Y chromosome

  • Males are haploid for the X chromosome, females are diploid

Sex determination in humans1
Sex Determination in Humans Alleles

  • Sex is determined by the Y chromosome

  • Up until about 7 weeks, an embryo has a uncommitted reproductive duct system

  • This duct system will develop into testes/penis if a Y chromosome is present

  • This is due to the expression of the SRY gene located on the Y chromosome

  • No Y chromosome duct system forms into ovaries/uterus

  • Faulty SRY  phenotypic female, sterile

Human inheritance patterns
Human Inheritance Patterns Alleles

  • X-linked recessive-gene is located on the X chromosome;2 copies of gene required for expression in females, 1 copy in males results in expression. XX vs XY

  • Ex. Fragile X syndrome, hemophilia, color blindness, muscular dystrophy, Menkes syndrome, adrenoleukodystropy

X linked recessive inheritance problem
X-linked Recessive Inheritance Problem Alleles

  • What type of offspring would a colorblind man and woman who is a carrier for CB have?

X linked recessive inheritance problem1

X-linked Recessive Inheritance Problem Alleles

Female-Cc not colorblind Colorblind male-c

But CB is X-linked XCXc XcY

Xc Y



Female offspring ratio 1 CB: 1 no CB

Male offspring ratio 1 CB: 1 no CB

X linked recessive inheritance problem2
X-linked Recessive Inheritance Problem Alleles

  • Can a female with muscular dystrophy ever have a son who does not have MS?

  • Can a male with hemophilia have a daughter who is not affected with the disease?

X linked recessive genetics problem

X-linked Recessive Genetics Problem Alleles

M-no MS m-has MS




No, all male offspring would have MS

X linked recessive genetics problem1

H-no Hemophilia h-Hemophilia Alleles

X-linked Recessive Genetics Problem

Xh Y



Yes, female offspring would not have hemophilia, but are carriers for the gene