key concept mendel’s research showed that traits … · 6.3 mendel and heredity key concept...

6.3 Mendel and Heredity

KEY CONCEPT Mendel’s research showed that traits are inherited as discrete units.

11.1 The Work of Gregor Mendel


Mendel laid the groundwork for genetics.

• Genetics is the study of biological inheritance patterns and variation.

• Traits are characteristics that are inherited.

• Traits are determined by genes carried on the chromosomes.

• Gregor Mendel is the father of genetics.



• Mendel used pollen to fertilize selected pea plants.

– interrupted the self-pollination process by removing male flower parts

Mendel’s data revealed patterns of inheritance.

Mendel controlled thefertilization of his pea plantsby removing the male parts,or stamens.

He then fertilized the femalepart, or pistil, with pollen froma different pea plant.



F2Generation(second filial)

3:175%purple-flower

25%white-flower

• Looking closer at Mendel’s work

P generation(parent)

F1generation(first filial)

100%purple-flower

Xpurple-flower white-flower

self-pollinate



• Mendel allowed the resulting plants to self-pollinate.

– Among the F1 generation, all plants had purple flowers– Among the F2 generation, some plants had purple

flowers and some had white



• Mendel observed patterns in the first and second generations of his crosses.



• Mendel’s findings:– There was a “factor” affecting the color of the

flowers that is passed from one generation to the next.

– The “factor” was later found to be the “gene”.– Each gene is made up of any combination of two

alleles (one from each parent) to produce different characteristics.

- Alleles could either dominant orrecessive.

purple white

alleles = different forms of a gene



Mendel’s three laws of inheritance:

1) Law of Dominance- dominant allele is always expressed; represented byuppercase letter

- recessive allele is expressed only when dominant allele is not present; represented bylowercase letter

ex: human eye colorbrown eye is dominant = “B”blue eye is recessive = “b”



Trait Dominant Recessive

Hairline Widow’s peak hairline

Straight hairline

Dimples Dimples No dimples

Earlobes Detached Earlobe

Attached Earlobe

Freckles Freckles No freckles



Trait Dominant Recessive

Little Fingers

Straight little fingers

Bent little fingers

Toes Big toe longer than second toe

Second toe longer than big toe

Tongue Roller No roller

Thumb Straight thumb

hitch‐hikers thumb



Mendel’s three laws of inheritance:2) Law of Segregation

- alleles separate during meiosis



Mendel’s three laws of inheritance:

3) Law of Independent Assortment- genes that segregate independently do notinfluence each other’s inheritance.

- helps account for the many genetic variationsobserved in many sexually reproducing organisms

ex: In Mendel’s experiment, plant seed color is notaffected by plant seed shape when both traitsare crossed. (Traits can mix and match!)- some are yellow and round- some are green and round- some are yellow and wrinkled- some are green and wrinkled



KEY CONCEPT Genes encode proteins that produce a diverse range of traits.

trait (or phenotype)

is determined by

gene (or genotype)

is made up of

2 alleles

may be

dominant recessive

11.2 Probability and Punnett Squares


The same gene can have many versions.

• All of an organism’s genetic material is called the genome.

• A gene is a piece of DNA that directs a cell to make a certain protein.

• Each gene has a locus, aspecific position on a pair ofhomologous chromosomes.

locus for seed shape gene



Genes influence the development of traits. • A genotype is the actual genetic make up of a trait.

- is made up of 2 alleles (one from each parent)ex: R = dominant allele; r = recessive allele

(Always use the same letter for each trait!!)- when same alleles are combined

homozygous or purebred - when different alleles are combined

heterozygous or hybrid- 3 possible genotypes:1) RR (homozygous dominant)2) rr (homozygous recessive)3) Rr (heterozygous)



Genes influence the development of traits. • A phenotype is the physical appearance of a trait.

- is determined by genotypeex: R = round seed (dominant)

r = wrinkled seed (recessive)

Genotype Phenotype

RR Round seed

rr Wrinkled seed

Rr Round seed

different genotypes can show same phenotype



Punnett squares illustrate genetic crosses.

• The Punnett square is a grid system for predicting all possible genotypes resulting from a cross.– The axes represent

the possible gametesof each parent.

– The boxes show thepossible genotypesof the offspring.

• The Punnett square yields the ratio of possible genotypes and phenotypes.



A monohybrid cross involves one trait.

• Monohybrid crosses examine the inheritance of only one specific trait.

• Use monohybrid crosses to explain Mendel’s experiments:- P = purple flower (dominant)- p = white flower (recessive)



• Mendel’s parent generation

PP x pp

P Pmale / sperm

p

pfem

ale

/ egg

s

%genotype

%phenotype

Pp Pp

Pp Pp

Pp

Pp

Pp

Pp

100%Pp

100%purple



• Mendel’s F1 generation

Pp x Pp

P pmale / sperm

P

pfem

ale

/ egg

s

PP75%

purple

25%white

3:1

25%PP

50%Pp

25%pp

1:2:1

%genotype

%phenotype

PP Pp

Pp pppp

Pp

Pp

1stgeneration(hybrids)

Aaaaah,phenotype & genotypecan have different

ratios!!



Practice Problem:In guinea pigs, black fur (B) is dominant to white fur (b). A heterozygous black guinea pig is crossed with a white guinea pig. Give the percentages or ratios of the genotypes and phenotypes of the offspring.

Bb

Bb

bb

bb

b

b

B b

Bb x bb

Genotype:50% Bb, 50% bb

Phenotype:50% black, 50% white



• A testcross is a cross between an organism with an unknown genotype and an organism with the recessive phenotype.

Example:P = pink flowers (dominant); p = white flowers (recessive)

Determine the genotype of Plant A that has pink flowers by using a test cross.

Cross Plant A with Plant B from the same species which has white flowers of known genotype pp.



• If plant A is homozygous dominant (PP):

Pp

Pp

Pp

Pp

p

p

P P

Genotype:100% Pp

Phenotype:100% pink

• If plant A is heterozygous (Pp):

Pp

Pp

pp

pp

p

p

P p

Genotype:50% Pp, 50% pp

Phenotype:50% pink, 50% white

You get all pink flowers in the offspring

You get 50-50 chance of getting pink or white flowers in the offspring



KEY CONCEPT The inheritance of traits follows the rules of probability.

Dominant Recessive

11.3 Exploring Mendel’s Genetics


A dihybrid cross involves two traits.

• Mendel’s dihybrid crosses led to his second law,the law of independent assortment.

• The law of independent assortment states that allele pairs separate independently of each other during meiosis.(This is why traits can mix and match!!)

• Example: seed color and seed shape



• Sample Dihybrid CrossTwo traits of apple:

1) Color = R (red) and r (green)2) Taste = S (sweet) and s (sour)

An apple heterozygous red and sour (__________) crosses with an apple green and heterozygous sweet (___________).

Rs Rs rs rs

Rrss x rrSs

Rrss

rrSs

x rS rs rS rspossible gametes produced

(always one allele from each trait!!)



Rs Rs rs rs

rS

rs

rS

rs

• Sample Dihybrid Cross



RrSs

Rrss

RrSs

Rrss

RrSs

Rrss

RrSs

Rrss

rrSs

rrss

rrSs

rrss

rrSs

rrss

rrSs

rrss

Genotypes:

RrSs =1/4Rrss =1/4rrSs =1/4rrss =1/4

Phenotypes:

Red & Sweet =1/4Red & Sour = 1/4Green & Sweet = 1/4Green & Sour = 1/4

Rs Rs rs rs

rS

rs

rS

rs

• Sample Dihybrid Cross

Rrss x rrSs



• Practice Problems

1) John cannot roll his tongue, but both of his parents can roll their tongue. Give the genotype of John and his parents. (C= can roll tongue; c= cannot roll tongue)

John = ccDad = CC or CcMom = CC or Cc

XX

Because John has to get one “c” from Dad and one “c” from Mom, both parents need to have one “c” to give. Therefore, the parents’ genotype is “Cc” (heterozygous).



• Practice Problems

2) A tall yellow plant is crossed with a tall green plant. Some of the offsprings are short and yellow. What are the parents’ genotypes? (T= tall, t= short; G= green, g= yellow)

Offspring = ttggParent 1 = TTgg or TtggParent 2 = TTGG or TTGg or TtGG or TtGgX

XXX


Because the offspring has to get one “t” and one “y” from each parent, both parents need to have one “t” and one “y” to give. Therefore, the parent 1’s genotype is “Ttgg” and parent 2’s genotype is “TtGg”.


• Most traits occur in a range and do not follow simple dominant-recessive patterns.

• Mendel worked with a simple system:- peas are genetically simple - most traits are controlled by single gene- each gene has only 2 versions:

1 completely dominant (A)1 recessive (a)

• But it’s usually not that simple!

Extending Mendelian Genetics



Exceptions to Mendel’s Laws:1) Incomplete Dominance

- Neither allele is completely dominant nor recessive.- Heterozygous phenotype is intermediate between the

two homozygous phenotypes.



• Sample Incomplete dominance Cross:

R rmale / sperm

R

r

female

/ eg

gs RR

Rr rr

Rr

25%

1:2:1

25%

50%

25%

1:2:1

%genotype

%phenotype

RR

Rr

Rr

rr

25%

50%

Rr x Rr



Exceptions to Mendel’s Laws:2) Codominance

– Both dominant and recessive alleles are expressed together.



• The ABO blood types result from codominant alleles.

- There are four blood types in human: A, B, AB, and O.- Alleles A and B are dominant to O. - Blood type AB carries both alleles A and B and shows codominance.

IoIo or ii



Exceptions to Mendel’s Laws:

3) Multiple Alleles– Many genes have

more than two alleles.– There are 3 different

alleles for human ABO blood types.

– Any person can carry up to two of the three possible alleles.

– Alleles are carried on the “I” gene on the red blood cell.

IoIo or ii

(A and B = dominant; o = recessive)



• Sample Multiple Allele Cross:

IA IO

IO

IO

IAIO

IAIO IOIO

IOIO

1:11:1

%genotype

%phenotype

50%Type A

IAIO x IOIO

50%IAIO

A heterozygous Type A man marries a Type O woman. What are the possible genotypes and phenotypes of their kids?

IOIO50% 50%

Type O



Many genes may interact to produce one trait.

• Polygenic traits are produced by two or more genes.

Order of dominance: brown > green > blue.


key concept mendel’s research showed that traits … · 6.3 mendel and heredity key concept...

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