Chapter 2: Traits and How They Change

Section 2: Genetics

Heredity x GeneticsMendel’s experimentsPunnett Square

For the test: study the powerpoint and textbook

1)What is Heredity? 2)What is genetics?

Heredity is the passing of traits from parents to offspring.

** Genetics is the branch of Biology that studies genes, heredity, and the variation of inherited traits in living organisms.

Mendel – Austrian monk, lived in the 1800s – The Father of Genetics

▶ He was the first to describe the inheritance of traits in an offspring, by studying pea plants.

▶ Why pea plants?

Pea plants reproduce quickly, sothe inheritance of traits can be observed in a short period of time

▶ As a result of his observations, Mendel developed 3 basic principles of genetics

Read pages 45-46 Mendel’s Principles of Heredity

▶ 1st Principle of Dominance▶ 2nd Principle of Segregation▶ 3rd Principle of Independent

Assortment

Traits are determined by different factors.Each trait is determined by at least two factors. One factor is dominant over the other

Today we call factors genes and alleles

1st PrincipleMendel’s Principle of Dominance

What are dominant and recessive alleles?

Dominant alleles will show their effect on the phenotype whenever they are present in the genotype. Recessive alleles will show their effect on the phenotype only when two of them are present in the genotype.

2nd Principle:Mendel’s Principle of Segregation Mendel also concluded that for each trait: Organisms receive ONE allele from the father and ONE from the mother.

This allows for many different allele combinations AND explains why variation exists among the offspring of the same parents.

Mendel’s 3rd principle :Law of Independent assortment

▶ Alleles for one trait have no effect on the inheritance of alleles of another trait

Vocabulary not in the book!

Homozygous or Purebred – an organism that has two identical alleles for a trait

Heterozygous or Hybrid – an organism that have two different alleles for a trait

**Representing alleles▶ Letters are used to represent alleles▶ Capital letters are used for dominant alleles and

lowercase letters are used for recessive alleles▶ Ex: short and tall – tall is dominant and short is

recessive▶ we always use the first letter of the dominant trait

TT homozygous or purebred = organism is tall tt homozygous or purebred = organism is shortTt heterozygous or hybrid = organism is tall (it has

the dominant allele, that will always show in the phenotype)

Punnett Square

▶ Tool to predict the genotype and phenotype of an offspring

Ex: cross a pink flower with a white flower. Pink is dominant

▶ Steps:1) Represent alleles2) Do the Punnet square3) Interpret the results in % for the

genotypes and phenotypes

More examples: Mendel’s experiments :

Mendel named: P - Parental generation F1 generation - the result of 1st crossing F2 generation - result of the 2nd crossing

****IMP: No matter what trait he was working with, Mendel observed that there was a pattern in the inheritance of traits.

Discovering the Patterns:EX: Tall and short plants – tall is dominant

-Cross two homozygous for the dominant trait -Cross two homozygous for the recessive trait -Cross two heterozygous

Discovering the Patterns: RESULTS BELOW ARE ALWAYS THE SAME NO MATTER THE TRAIT

-Cross two homozygous for the dominant trait = 100% with the dominant trait

-Cross two homozygous for the recessive trait = 100% with the recessive trait

-Cross two heterozygous = 75% with the dominant trait and 25% with the recessive trait

Practicing Punnett Square

- Pea plants with different seed colorDominant – yellow seed – what is the genotype?Recessive – green seed – what is the genotype? 1)Homozygous yellow x homozygous green2)Heterozygous yellow x heterozygous yellow3)green x green4)Heterozygous yellow x green5)Homozygous yellow x homozygous yellowIdentify Genotypes and phenotypes for all crossesPage 47 – Applying math

Non-Mendelian Inheritance – not in the book – use powerpoint to study

▶ The inheritance of traits not always follows Mendel's rules

▶ Mendel studied traits determined by one gene with 2 pair of alleles and one allele is always completely dominant over the other

▶ This type of inheritance is called Complete Dominance

▶ Incomplete Dominance ▶ Codominance ▶ Multiple Alleles▶ Polygenic Inheritance▶ Sex-Linked Traits

Nowadays we know that there are other types of inheritance:

▶ traits determined by genes with alleles that are neither dominant nor recessive .

▶ The offspring show a phenotype that is a blend of the parents phenotype (pink flower)

Incomplete dominance- Snapdragon

▶ Snapdragons:

A plant that is homozygous for the red allele will have red flowers

A plant that is homozygous for the white allele will have white flowers

A plant that is heterozygous will have pink flowers

Neither the red nor the white alleles are dominant, so the phenotype of the offspring is a blend of the two parents.

Incomplete dominance- Snapdragon

Representing the alleles: use the upper case letter of the trait = colorCR CR – redCW CW – whiteCR CW – pink

Incomplete dominance- Snapdragon

Punnett Square : CR CR – red versus CW CW – whiteCR CW – pink versus pink

Incomplete Dominance in horses - color of the fur

CCCC

CWCW

CCCW

- Chestnut – homozygous- White – homozygous - Palomino – heterozygous

Incomplete Dominance in horses

CCCC

CWCW

CCCW

- Chestnut - homozygous- White – homozygous - Palomino – heterozygous

Do a Punnett Square to cross: Palomino with PalominoPalomino with Chestnut

Incomplete dominance in humansWavy hair – inherit curly hair from a parent and straight hair from another parent

▶ Codominance is a type of incomplete dominance.

▶ Codominance would show both alleles equally. It is not a blend of the traits as is seen in incomplete dominance.

▶ alleles are represented as in Incomplete Dominance

Codominance

Blood Type AB in humans is a case of CodominanceBlood types are: A,B,AB,O

Blood AB:A and B are proteins called antigens, present in the blood In Blood AB - both proteins are present in the phenotype. (not a blend)

worksheet

▶ The trait is determined by a gene that has more than 2 alleles.

▶ Blood type is an example▶ There are:2 dominant alleles: alleles for type A and

alleles for type BOne recessive for type O

Multiple Alleles: another type of inheritance

Blood type alleles and how to represent them:

i – recessiveIA and IB are Codominant(when present,both will show in the phenotype – Type AB)

Cross type AB with Type O

50% A heterozygous50% B heterozygous

Cross heterozygous A with heterozygous BCross heterozygous A with recessive O

▶ When traits are determined by more than one gene.

▶ EX: height, eye color, skin color ▶ More difficult to study because of the

wide variety of phenotypes (range) that the different gene combinations can produce

Polygenic Inheritance

SKIN COLOR: 3 Genes and 6 Allelesthe skin gets lighter when more recessive alleles are present in the genotype

Different genotypes and phenotypes

▶ X and Y chromosomes determine the sex▶ Remember, if you have: ▶ 2 chromosomes X – female (XX)▶ 1 chromosome X and one Y – male (XY)▶

Sex-Liked traits - Remember

▶ are traits linked with the X or Y chromosomes

▶ linked means that the chromosome has the allele for that trait, so the trait is inherited with the X or Y chromosome

▶ Sex linked traits are usually X-linked ▶ Y-linked traits – very rare▶ Ex of sex-linked traits: Color-blindness,

hemophilia – both recessive traits

What are Sex-Liked traits?

▶ Conditions for the recessive trait to show in the phenotype:

Females need to be homozygous to have the disease, males have to have only one chromosome to be affected by the disease.

Sex-Linked traits to the X chromosome:

The alleles in the Sex-linked traits ▶ EX: Hemophilia –bleeding disorder that slows

the blood clotting process – it is recessive

▶ To be hemophilic – women need two lower case h and man needs one lower case h

The alleles in the Sex-linked traits ▶ EX: Colorblindness - it is recessive

▶ to be color blind – woman needs 2 lower case c and male needs 1 lower case c.

What is a carrier?

Is an organism that carries the allele for a certain trait but does not have the trait expressed in the phenotype.