+Intro to Genetics and MendelHonors Biology

Ms. Kim

+Transmission (passing down) of

Traits

How?

One possible explanation of heredity is a “blending” hypothesisgenetic material contributed by two parents mixes

+Another Hypothesis

An alternative to the blending model is the hypothesis of inheritance (genes) Parents pass on discrete heritable units (factors) called genes

http://wps.aw.com/bc_campbell_biology_7/29/7523/1925929.cw/nav_and_content/index.html The Novelty Gene Video

+ Gregor Johann Mendel (1843)

Austrian Monk- “Father of Modern Genetics”

Documented a mechanism of inheritance through his experiments with garden peas

The scientific study of heredity is called GENETICS!

Worked with pea plants in his monastery

Correctly believed that heritable factors (genes) retain their individuality from generation to generation i.e. – marbles (no blending of colors!)

Figure 14.1

+Gregor Johann Mendel

Mendel used the scientific method to identify two laws of inheritance

Mendel discovered the basic principles of heredityBy breeding garden peas in carefully planned (CONTROLLED) experiments

+ Mendel’s Experimental Method

Why did Mendel choose pea plants? available in many varieties of traits

They have seven distinct & observable traitseasy to gethe could strictly control which plants

mated with which Grow quickly

They reproduce quickly & have a short life cycleThey produce many offspring in one cross

Mendel’s Experimental CrossPurebred white

and purple flowers

White flowers reappear in

some offspring

Offspring were allowed to self

pollinate

What did Mendel notice? Did the trait for white flowers disappear in F1 generation?

A. All PurpleB. All WhiteC. Both

White/Purple

+Some genetic vocabulary

Character: a heritable feature, such as flower color

Trait: a variant of a character, such as purple or white flowers

+

Mendel observed the same patternIn many

other pea plant characters

+Pea Plant Fertilization

Self fertilization : mate with self produce identical offspringTRUE or PURE breeds

Cross fertilization : mate with another can produce different offspringHYBRIDS http://wps.aw.com/bc_campbell_biology_7/29/7523/1925929.cw/nav_and_content/index.h

tml Colored Cotton Video

+Mendel’s Experiment

Mendel only looked at “either-or” charactersEx: Purple OR white flowers

Mendel started his experiments with “true-breeding”Made through self fertilization so plants are “TRUE” for only 1 traitKnown as HOMOZYGOUS for trait

+What was Mendel’s

Procedure?1. He made 14 “TRUE

BREEDS” 1 for EACH trait

he looked at

These are the original parents Are called the

P generation

+What was Mendel’s

Procedure?2. He used cross fertilization to mate 2 true breeds for same gene

Ex: Purple vs white flower color

3. He collected the offspring (progeny)

The hybrid (mixed) offspring of the P generation

Are called the F1 generation

+What was Mendel’s

Procedure?

4. He crossed (using cross fertilization) male and female from F1 progeny

When F1 individuals are mated together

The F2 generation is produced

+What did Mendel Discover?

A 3:1 ratio, purple to white flowers, in the F2 generationP Generation

(true-breeding parents)

Purpleflowers

Whiteflowers

F1

Generation (hybrids)

All plants hadpurple flowers

F2 Generation

Where did the white color go?

P F1 F2

+

17+Genetics Vocab

Mendel worked with his pea plants until he was sure that all were true-breeding varieties (pure bred)

P generation parental generation have offspring called the F1 generation (hybrids)

Hybrid (F1) the offspring of two true breeding varieties

If F1 generations self-fertilize/cross, their offspring are called the F2 generation

+What are Mendel’s factors?

Mendel’s “factors” are now called allelesAlternative version or form of a gene

Figure 14.4

Allele for purple flowers

Locus for flower-color gene Homologouspair ofchromosomes

Allele for white flowers

F

f

19+Mendel’s Experiments

After studying pea plants, Mendel concluded that:Traits are passed from one generation to the

next through genes.Each trait is controlled by a different form of a

gene called an alleleSome alleles are dominant to others called recessive traits

New question: Have the recessive alleles disappeared or are they still present in the parents?

+What did Mendel Conclude?

Mendel reasoned thatIn the F1 plants, only 1 factor (ex: purple flower) was affecting physical outcome color in hybrids

This factor was dominant and the hidden factor was recessive

+Recessive is…

Represented by a lowercase letter (it is NOT the letter itself, though)

an allele that does NOT produce a characteristic effect when present with a dominant allele

only expressed when present with another (identical) recessive allele This is known as the homozygous

conditionaa or hh

+Dominant is…

Represented by a uppercase letter

an allele that produces the same trait whether inherited with a another dominant allele (homozygous) or with a recessive allele (heterozygous)Aa or AA

The allele that is expressed if present

+ Frequency of Dominant Alleles

Dominant allelesNot necessarily better, stronger, etc. than recessive alleles

Ex: Polydactyl

24+Mendel’s Experiments

Mendel crossed the first generation and saw that the recessive trait showed up in about 1 of 4 plants.

Conclusion: Law of Segregation!

What is the Law of Segregation?!

Organisms inherit two copies of each gene (one from each parent)

Organisms donate only one copy of each gene in their gametes (sex cells)

Therefore, the two copies of each gene segregate (separate) during gamete formation (meiosis)

+The Law of Segregation:

Mendel’s 1st LawEach gamete ONLY gets 1 allele

+ More about Alleles…

Each individual has 2 alleles for the same gene located on homologous chromosomes

Each parent passes 1 allele for each gene to his/her offspringIn sperm or egg

What stage of meiosis are alleles segregated into gametes?Meiosis Anaphase I

+

a A

28+Mendel’s Observations

Used pea plants to see patterns in the way various traits were inherited

Using his data, he saw that different traits are inherited separately Example: Green pea color isn’t always inherited

with wrinkled pea shape Green peas can be smooth and round too!

This is called the Law of Independent Assortment!

29+What is the Law of Independent Assortment?

Allele pairs (traits) separate independently of each other during gamete formation (meiosis) Different traits are inherited

separately Example – peas can be green and

wrinkled OR green and roundThis explains genetic diversity among organisms

+ Law of Independent Assortment:Mendel’s 2nd Law

Says genes are inherited independently of other genesGenes are not linked unless on the same chromosome!

Mendel assumed traits occur on different chromosomes!

Occurs during Metaphase I

+ Useful Genetic Vocabulary

Homozygous A pair of IDENTICAL (same) alleles for that

geneExhibits true-breeding

aa = homozygous recessive (or just recessive)

HH = homozygous dominant

Heterozygous Pair of alleles that are different for that gene

Aa or Hh

+More Genetic Vocabulary

An organism’s genotypeIs its genetic (DNA) makeupA.k.a.-the allele combination (includes 2 alleles)

An organism’s phenotypeIs its physical outcome of the genotypeEx: blue eyes or AB blood type

+ Mendelian Genetics…aka- COMPLETE DOMINANCE

If an organism is heterozygous (Hh),The effect of the recessive allele is HIDDEN

Heterozygous and homozygous dominant have SAME phenotype

The 1st allele is “completely dominant” over the 2nd allele

+

Figure 14.6

3

Phenotype

Purple

White

Genotype

PP(homozygous)

Pp(heterozygous)

pp(homozygous)

Ratio 3:1 Ratio 1:2:1

Purple

Purple

1

1

1

2Pp

(heterozygous)

+ Why Did Mendel Keep Getting the SAME results?

We can answer this question using a Punnett squarea diagram (box) used to predict probabilities of possible outcomes for offspring that will result from a cross between 2 parentsSHOWS EXPECTED RESULTS (not necessarily actual)

+Practice

PURPLE X WHITE

PURPLE

PURPLE & WHITE

Which flower color is recessive?A. WhiteB. PurpleC. NeitherD. Both

+Practice

PURPLE X WHITE

PURPLE

PURPLE & WHITE

Which flower color is recessive? WHITE

What would the genotype be for the recessive flower?

A. PP homozygous dominantB. pp homozygous recessiveC. Pp Heterozygous

+Practice

PURPLE X WHITE

PURPLE

PURPLE & WHITE

Which flower color is recessive? WHITE

What would the genotype be for the recessive flower? pp homozygous recessive

Which flower color is dominant?A. WhiteB. PurpleC. NeitherD. Both

+Practice

PURPLE X WHITE

PURPLE

PURPLE & WHITE

Which flower color is recessive? WHITE

What would the genotype be for the recessive flower? pp homozygous recessive

Which flower color is dominant? PURPLE

What would the genotype be for the dominant flower color?

A. PP homozygous dominantB. Pp heterozygous C. pp homozygous recessiveD. Both A & B

40+Genetics Vocab (pt 2)

Monohybrid cross cross where parents differ in only one trait (Rr x rr)

Dihybrid cross cross where parents differ in two traits (RrHh x rrHH)

Punnett square – a diagram that shows the gene combinations that might result from a genetic cross of two parents