Mindarie Senior College

2A/2B Human Biological

Science

Genetic VariationGenetic Variation

Mendelian Patterns of Mendelian Patterns of InheritanceInheritance

Gregor MendelGregor Mendel Born in the Czech Republic in 1822. Born in the Czech Republic in 1822. Died in 1884 with his work still Died in 1884 with his work still

unnoticed.unnoticed. Worked in an Austrian monastery as a Worked in an Austrian monastery as a

monk/teacher/scientist/gardener.monk/teacher/scientist/gardener. Experimented with purebred tall and Experimented with purebred tall and

short peas.short peas. Discovered some of the basic laws of Discovered some of the basic laws of

heredity.heredity. Studied seven pure bred traits in peas.Studied seven pure bred traits in peas. Called the stronger hereditary factor Called the stronger hereditary factor

dominant.dominant. Called the weaker hereditary factor Called the weaker hereditary factor

recessive.recessive. Presentation to the Science Society Presentation to the Science Society

in 1866 went unnoticed. ie, no in 1866 went unnoticed. ie, no comtemporary recognitioncomtemporary recognition

Work and findings rediscovered in 1900 Work and findings rediscovered in 1900 by Hugo de Vries and Carl Correns by Hugo de Vries and Carl Correns

Known as the “Father of Genetics”.Known as the “Father of Genetics”. Moral: you too can be famous!Moral: you too can be famous!

Mendel’s ObservationsMendel’s Observations He noticed that peas are easy to He noticed that peas are easy to

breed for pure traits and he called the breed for pure traits and he called the pure strains purebreds.pure strains purebreds.

He developed pure strains of peas for He developed pure strains of peas for seven different traits (i.e. tall or short, seven different traits (i.e. tall or short, round or wrinkled, yellow or green, round or wrinkled, yellow or green, etc.)etc.)

He crossed these pure strains to He crossed these pure strains to produce hybrids.produce hybrids.

He crossed thousands of plants and He crossed thousands of plants and kept careful records for eight years.kept careful records for eight years.

Mendel’s PeasMendel’s Peas In peas many traits appear in two forms (i.e. tall or In peas many traits appear in two forms (i.e. tall or

short, round or wrinkled, yellow or green.)short, round or wrinkled, yellow or green.) The flower is the reproductive organ and the male The flower is the reproductive organ and the male

and female are both in the same flower.and female are both in the same flower. He crossed pure strains by putting the pollen (male He crossed pure strains by putting the pollen (male

gamete) from one purebred pea plant on the pistil gamete) from one purebred pea plant on the pistil (female sex organ) of another purebred pea plant to (female sex organ) of another purebred pea plant to form a hybrid or crossbred plant. form a hybrid or crossbred plant.

Mendel’s ResultsMendel’s ResultsMendel crossed purebred tall plants with purebred short plants and the first generation plants were all tall.

When these tall offspring were crossed the result was a ratio of 3 tall to 1 short.

Mendel’s PeasMendel’s Peas

Mendel’s ExperimentsMendel’s Experiments

He He experimentally experimentally crosses different crosses different strains to strains to develop hybrids.develop hybrids.

He then crossed He then crossed the hybrids and the hybrids and analyzed the analyzed the results.results.

Dominant Traits Dominant Traits RULERULE

Strong Hereditary Strong Hereditary traits cover weak traits cover weak traits.traits.

Mendal called Mendal called stronger traitsstronger traits DOMINANTDOMINANT

Mendal called Mendal called weaker traitsweaker traits recessiverecessive

Dominant traits Dominant traits are represented are represented by capital letters by capital letters (T)(T) while while recessive traits recessive traits are represented are represented by lower case by lower case letters letters (t).(t).

Important terms used in Important terms used in geneticsgenetics

TermTerm MeaningMeaning

MonohybrMonohybrid crossid cross

The inheritance of a genetic feature The inheritance of a genetic feature controlled by a single pair of controlled by a single pair of genes. (monohybrid = an organism genes. (monohybrid = an organism that is hybrid for one genetic that is hybrid for one genetic feature)feature)

AlleleAllele The different forms of a gene. Most The different forms of a gene. Most genes have only two alleles (e.g. genes have only two alleles (e.g. the height of the pea plants in the the height of the pea plants in the above example was controlled by a above example was controlled by a gene, which had a tall allele and a gene, which had a tall allele and a short allele). Although some genes short allele). Although some genes have more than two alleles a have more than two alleles a hybrid individual can only possess hybrid individual can only possess two of them.two of them.

HomozygousHomozygous Where both alleles Where both alleles determining a genetic determining a genetic feature are the same feature are the same (e.g. TT)(e.g. TT)

HeterozygousHeterozygous Where the two alleles Where the two alleles determining a genetic determining a genetic feature are different feature are different (e.g. Tt)(e.g. Tt)

GenotypeGenotype The combination of The combination of genes that determine a genes that determine a character, whether or character, whether or not they are expressed not they are expressed (e.g. Tt)(e.g. Tt)

PhenotypePhenotype The physical The physical expression or expression or appearance of a appearance of a particular genotype particular genotype (e.g. the physical (e.g. the physical expression or expression or phenotype of a pea phenotype of a pea plant with the plant with the genotype Tt will be a genotype Tt will be a plant that is tall).plant that is tall).

AutosomeAutosome A non-sex chromosomeA non-sex chromosome

Sex Sex chromosomechromosome(X-(X-chromosome)chromosome)

The 23rd pair of The 23rd pair of chromosomes that chromosomes that determine the sex of determine the sex of an individual an individual

Mendel’s first law - The Law of Mendel’s first law - The Law of SegregationSegregation

This law states that: This law states that:

allele pairs separate or segregate allele pairs separate or segregate during gamete formation, and during gamete formation, and randomly unite at fertilizationrandomly unite at fertilization

The underlying The underlying assumptionsassumptions

The Law of Segregation is based on the assumption The Law of Segregation is based on the assumption that: that:

There are alternative forms for genes (known as There are alternative forms for genes (known as alleles)alleles)

For each characteristic, or trait, organisms For each characteristic, or trait, organisms inherit two alternative forms of that gene - one inherit two alternative forms of that gene - one from each parentfrom each parent

When gametes (sex cells) are produced, allele When gametes (sex cells) are produced, allele pairs separate leaving the gametes with a pairs separate leaving the gametes with a single allele for each traitsingle allele for each trait

When the two alleles of a pair are different, one When the two alleles of a pair are different, one is dominant and the other is recessiveis dominant and the other is recessive

Mendel’s hypothesis - 1Mendel’s hypothesis - 1

Mendel predicted that if he cross-Mendel predicted that if he cross-pollinated true-breeding long-stemmed pollinated true-breeding long-stemmed plants (homozygous dominant) with plants (homozygous dominant) with true-breeding short-stemmed plants true-breeding short-stemmed plants (homozygous recessive), all of the (homozygous recessive), all of the resulting offspring (the F1 generation) resulting offspring (the F1 generation) would be long-stemmed.would be long-stemmed.

Dominant allele(long stem)

Recessive allele(short stem) TT

T

tt

t

Tt Tt Tt Tt

The gametes or sex cells

Tall Tall Tall Tall

Tall

Short

Calculating autosomal Calculating autosomal dominant-recessive patterns dominant-recessive patterns of inheritanceof inheritance Let the allele for long stems be represented by Let the allele for long stems be represented by

TT Let the allele for short stems be represented by Let the allele for short stems be represented by

ttParental Parental phenotypephenotype

Tall x ShortTall x Short

Parental Parental genotypegenotype

TT x ttTT x tt

Possible Possible gametesgametes

T x t onlyT x t only

F1 genotypeF1 genotype All TtAll Tt

F1 phenotypeF1 phenotype All tallAll tall

Punnett SquaresPunnett Squares Predicting the result of a genetic Predicting the result of a genetic

cross is made easier by using a cross is made easier by using a Punnett square.Punnett square.

T

T

t

t

TT Tt

Tt tt

Genotype of first parent

Genotype of second parent

Predicted genotypes of offspring

* * Statistical predictionsStatistical predictions

The result of genetic crosses only The result of genetic crosses only shows the statistical probability of an shows the statistical probability of an offspring having a particular offspring having a particular genotypegenotype

ExampleExample:: The chance of a baby being a particular The chance of a baby being a particular

sex is ½ male: ½ female (or 1:1).sex is ½ male: ½ female (or 1:1).This means that EVERY baby has an This means that EVERY baby has an equal chance of being a boy or a girl – if equal chance of being a boy or a girl – if the first baby is a boy the second baby the first baby is a boy the second baby has a 50% chance of also being a boy.has a 50% chance of also being a boy.

The predicted ratio can be seen best in The predicted ratio can be seen best in large populations, such as Mendel’s pea large populations, such as Mendel’s pea experiments, which involved thousands experiments, which involved thousands of plants.of plants.

Some Mendelian traits in Some Mendelian traits in HumansHumans

Ability to taste PTCAbility to taste PTC Albinism Albinism Blood typeBlood type Brachydactyly (Shortness of fingers and toes) Brachydactyly (Shortness of fingers and toes) Cheek dimplesCheek dimples Cleft chin (cleft=dominant; smooth=recessive) Cleft chin (cleft=dominant; smooth=recessive) Free or attached earlobes Free or attached earlobes Wet or dry earwax Wet or dry earwax Face frecklesFace freckles Hitch hiker’s thumbHitch hiker’s thumb Polydactyly (six fingers)Polydactyly (six fingers) Widow’s peak (Widow's peak=dominant, straight Widow’s peak (Widow's peak=dominant, straight

hairline=recessive) hairline=recessive) Tongue rollingTongue rolling