Mendelian Genetics

An Overview

Gregor Johann MendelGregor Johann Mendel•Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants•He found that the plants' offspring retained traits of the parents•Called the “Father of Genetics"

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Chromosomes and Genetics

• Genes are short regions of this DNA that hold the information needed to build and maintain the body

• Heredity- the passing of characters from parents to offspring

• Genetics- the branch of science that deals with heredity.

• Pea plants have several advantages for genetics. Several characters in garden plants exist in TWO clearly

different forms. Another advantage of peas is that Mendel had strict control over

which plants mated with which. Self fertilization- Each pea plant

has BOTH male & female reproductive

organs. (stamens) –male reproductive organ(pistil)- female reproductive organs.

Small Grows easily Matures quickly Produced many off-spring

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 14.1

The Seven Characters Mendel Studied

Character –heritable feature (physical features)

Trait – each variant for a character (dominant or recessive)

• Mendel’s initial experiments were Monohybrid crosses. Monohybrid Cross- a cross that involves one pair

of contrasting traits. True-breeding- plants capable of producing

offspring with only one form of a particular physical feature.

– True-breeding plant served as the P generation (Parent) and their hybrid offspring are the F1 generation.

• Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Ratios: Mendel’s Results• When Mendel allowed the F1 plants to self-fertilize, the

F2 generation included both purple-flowered and white-flowered plants.

– The white trait, absent in the F1, reappeared in the F2.

• Based on a large sample size, Mendel recorded 705 purple-flowered F2 plants and 224 white-flowered F2 plants from the original cross.

Mendel’s 4 Hypotheses1. Alternative versions of genes account for variations in inherited

characters, which are now called alleles1. concept of alleles (P=purple, p=white)

2. For each character an organism inherits two alleles, one from each parent. This set of alleles is called its genotype.

3. If the two alleles at a locus differ, the dominant allele is fully expressed in the organism’s appearance (phenotype).

4. The law of segregation - the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

Allele for purple flowers

Locus for flower-color gene

Homologous

pair of

chromosomes

Allele for white flowers

Homozygous- when the two alleles of a particular gene present in an individual are the same

Heterozygous-when the alleles of a particular gene present are different.

Mendel’s law of Segregation

The two alleles for a character

segregate (separate)

when gametes are formed.

Mendel’s Law of Independent Assortment

~The alleles of different genes separate independently of one another during gamete formation.

Vocabulary

• Character –heritable feature

• Trait – each variant for a character

• True-breeding – plants that self-pollinate all offspring are the same variety

• Monohybrid cross – a cross that tracks the inheritance of a single character

• P generation – (parental) true-breeding

• F1- (first filial) offspring of P generation

• F2 – (second filial) offspring from F1 cross

Genetics and Genetics and PredictionPrediction

Punnett Squares: Punnett Squares: ProbabilityProbability

ProbabilityProbability – predict likelihood of an – predict likelihood of an event or outcomeevent or outcome

Punnett squarePunnett square – grid for organizing – grid for organizing genetic informationgenetic information Can be used to make predictions about Can be used to make predictions about

a cross between two organsimsa cross between two organsims Monohybrid Cross – cross between – cross between

two parents and one traittwo parents and one trait

Rules to the Punnett Rules to the Punnett SquareSquare

1. Figure out the genotypes of the parents.

TT & tt

Mom Dad

Rules to the Punnett Rules to the Punnett SquareSquare

2.Figure out what kinds of gametes the parents can produce.

TT & tt

Mom Dad

T

T

t

t

Rules to the Punnett Square

3. Set up a Punnett Square for your mating.

Mom

Dad

T T

t

t

Tt Tt

Tt Tt

Rules to the Punnett Square

Genotype- genetic make-up (the alleles an organism has.)

Phenotype – Physical Features or characteristics.

Monohybrid CrossMonohybrid Cross

Predictions for Two Predictions for Two TraitsTraits

Dihybrid CrossDihybrid Cross – cross between two – cross between two parents and two traitsparents and two traits Use a 4 x 4 Punnett squareUse a 4 x 4 Punnett square Sixteen possible outcomesSixteen possible outcomes

Autosomal traits vs. Sex-Autosomal traits vs. Sex-linked traitslinked traits

Autosomes – body chromosomes – first Autosomes – body chromosomes – first 22 pair22 pair

Same in both male and femaleSame in both male and female

Sex Chromosomes – 1 pair, last pairSex Chromosomes – 1 pair, last pair

XX – FemaleXX – Female

XY - MaleXY - Male

Sex Linked TraitsSex Linked Traits

Traits controlled by recessive genes Traits controlled by recessive genes located on sex chromosomes (normally located on sex chromosomes (normally associated with the X chromosome)associated with the X chromosome)

HemophiliaHemophilia – blood clotting enzyme – blood clotting enzyme

is absentis absent Queen Victoria’s family affectedQueen Victoria’s family affected

Sex Linked TraitsSex Linked Traits

Red – Green ColorblindnessRed – Green Colorblindness – individuals – individuals cannot distinguish between these two cannot distinguish between these two colorscolors

XR

Xr

XR

XR XR

XR Xr

Y

XR Y

Xr Y

XR

Xr

Xr

XR Xr

Xr Xr

Y

XR Y

Xr Y

Females•XRXR = normal•XRXr = carrier•XrXr = disease

Males•XRY = normal•XrY = disease

Pedigree ChartsPedigree Charts Males = Males = Females = Females = Generations = Roman NumeralsGenerations = Roman Numerals Individuals = Numbered sequentiallyIndividuals = Numbered sequentially Trait Expressed = Filled inTrait Expressed = Filled in Non Carriers = EmptyNon Carriers = Empty Carrier (not ill) = Half filledCarrier (not ill) = Half filled

Individual who carries a recessive allele Individual who carries a recessive allele that is not expressedthat is not expressed

Pedigree ChartsPedigree Charts

Chart which shows how a trait and the Chart which shows how a trait and the genes that control it are inherited genes that control it are inherited within a familywithin a family

Identifies the presence or absence of Identifies the presence or absence of particular trait in members of each particular trait in members of each generationgeneration

Today... Pedigree analysisToday... Pedigree analysis

In humans, pedigree analysis is an important tool for studying inherited diseases

Pedigree analysis uses family trees and information about affected individuals to:

figure out the genetic basis of a disease or trait from its inheritance pattern

predict the risk of disease in future offspring in a family (genetic counseling)

Today... Pedigree analysisToday... Pedigree analysis

How to read pedigrees

Basic patterns of inheritance1. autosomal, recessive2. autosomal, dominant3. X-linked, recessive4. X-linked, dominant (very rare)

Applying pedigree analysis - practice

1.1. Autosomal recessive traitsAutosomal recessive traits

Trait is rare in pedigree

Trait often skips generations (hidden in heterozygous carriers)

Trait affects males and females equally

Most common ones

Cystic fibrosis Sickle cell anemia

Autosomal dominant traitsAutosomal dominant traits

There are few autosomal dominant human diseases (why?), but some rare traits have this inheritance pattern

Only need to get the trait from one parent to be affected.

ex. ex. achondroplasia achondroplasia

(a sketelal (a sketelal disorder disorder causing causing

dwarfism)dwarfism)

X-linked recessive pedigreesX-linked recessive pedigrees

Trait is rare in pedigree

Trait skips generations

Males are more often affected than females

Affected fathers DO NOT pass to their sons,

X-linked recessive traitsX-linked recessive traits

ex. Hemophilia in European royaltyex. Hemophilia in European royalty

X-linked dominant pedigreesX-linked dominant pedigrees

Trait is common in pedigreeTrait is common in pedigree

Affected fathers pass to ALL of their Affected fathers pass to ALL of their daughtersdaughters

Males and females are equally likely to be Males and females are equally likely to be affectedaffected

Beyond Mendelian Beyond Mendelian Genetics: Incomplete Genetics: Incomplete

DominanceDominanceMendel was lucky!Mendel was lucky!Traits he chose in theTraits he chose in thepea plant showed up pea plant showed up very clearly…very clearly…One allele was dominant over another, One allele was dominant over another,

so phenotypes were easy to so phenotypes were easy to recognize.recognize.

But sometimes phenotypes are not But sometimes phenotypes are not very obvious…very obvious…

Polygenic TraitsPolygenic Traits When a character (physical feature) is When a character (physical feature) is

influenced by several different genes, influenced by several different genes, the character is the character is Polygenic.Polygenic.

Examples: Height, weight, color of Examples: Height, weight, color of skin, hair, and eyesskin, hair, and eyes

Cause slight and often variable range Cause slight and often variable range of differences throughout populationof differences throughout population

Incomplete DominanceIncomplete Dominance

Snapdragon flowers come in many colors.Snapdragon flowers come in many colors.

If you cross a red snapdragon (RR) with a If you cross a red snapdragon (RR) with a white snapdragon (rr)white snapdragon (rr)

You get PINK flowers (Rr)!You get PINK flowers (Rr)! R R

R r

r r

Genes show incomplete dominance when the heterozygous phenotype

is intermediate.

Incomplete dominanceIncomplete dominanceWhen F1 generation (all pink flowers) is self

pollinated, the F2 generation is 1:2:1

red, pink, white

R R R r

R r

r r

R r

R

r

CodominanceCodominance Two alleles affect Two alleles affect

the phenotype in the phenotype in separate and separate and distinguishable distinguishable ways. ways.

Neither allele can Neither allele can mask the other and mask the other and both are expressed both are expressed in the offspring and in the offspring and not in an not in an “intermediate” “intermediate” form.form.

Multiple AllelesMultiple Alleles Genes with 3 or more Genes with 3 or more

alleles are said to alleles are said to have have Multiple Multiple alleles.alleles.

Blood type is determined by three alleles, IA, IB, and i.

The IA and IB alleles are both dominant over i, but neither IA, IB is dominant over the other (Codominance)

Blood Type

Environmental Impact on Environmental Impact on PhenotypePhenotype

pH of the soil will change the color of hydrangea flowers from blue to pink

Environmental Impact on Environmental Impact on PhenotypePhenotype

Genetic DisordersGenetic Disorders

A hereditary A hereditary mutationmutation is a mistake that is is a mistake that is present in the DNA of virtually all body present in the DNA of virtually all body cellscells

Genetic Disorders arise from mutationsGenetic Disorders arise from mutationsSickle Cell AnemiaSickle Cell AnemiaCystic fibrosisCystic fibrosisHemophiliaHemophiliaHuntington's DiseaseHuntington's DiseaseHypercholesterolemia Hypercholesterolemia