Genes & Traits Gene – section of DNA that encodes a

protein, resulting in/affecting a trait Genetics – study of heredity, or how

organisms inherit characteristics from parents

Trait – a characteristic of an organism; *genetic traits are inherited*Ex.: hair color, enzymes, size (potential), etc.

Genetics (ch. 9 & 12)

There once was an Austrian monk who liked peas and was

very good at math…

Gregor Mendel’s (1860s)Pea Plant Experiments

Mendel crossed pea plants with different traits and observed the results in the offspring.

Mendel’s Pea Plant Experiments

Dominant

Recessive

When two purebreds were crossed, recessive traits disappeared in the F1, reappeared in the F2.Mendel hypothesized that each trait was controlled by a “factor” andthat there must be at least two forms of each “factor.”

Why pea plants?•short life cycle•traits easily observed, no blending•self-fertilize easily, easy to manipulate/fertilize

Most genetic traits in nature are not so easy to distinguish.

Genes & Traits Allele – one form of a gene

Dominant allele – expressed whenever present, represented by a capital letter; ex.: B for brown hair color

Recessive allele – only expressed when two copies are present; represented by a lower case letter; ex.: b for blonde hair color

A diploid organism inherits two alleles (one from each parent) for every genetic trait

Genotype - the set of alleles for a trait or all traits of an organism

Genotypes

Purebred/Homozygous

Organism that has only one form of a gene/trait

(same allele received from both parents)

Hybrid/Heterozygous

Organism that has two forms of a gene/trait

(different alleles received from each parent)

BB – homozygous dominant genotype

bb – homozygous recessive genotype

Bb – heterozygous genotype

Phenotypes Phenotype - the trait the organism actually

expresses; ex.: having brown hair, or having blonde hair The phenotype may show on the appearance of the

organisms or not; ex.: being able to digest lactose is a phenotype that does not “show”

A phenotype results from the expression of a gene a protein is made that affects the phenotype

Phenotype Possible Genotypes

Dominant Ex.: brown hair

BB orBb

Recessive Ex.: blonde hair

bb

When we look at the genotypes in each generation:

Check yourself:

• Discuss with your partner:– Can you find an example of a dominant trait in

one of you?– Can you find an example of a recessive trait

in one of you?– What is the phenotype for each trait?– What are the possible genotypes for each

trait?

(not in notes)

GG gg

Gg Gg

GG or Gg gg

Mendel’s Laws (modern form) Law of Segregation - Gene pairs separate when

gametes form:Half of an organisms gametes contain one gene from a

homologous pair, half contain the other gene

Law of Independent Assortment - Different traits are inherited independently:

Genes for different traits segregate into gametes randomly and independently from each other new combinations You can inherit your father’s eyes and your mother’s smile!

Law of Dominance - Dominant alleles are expressed, recessive alleles can be hidden:

Hybrid pea plant: Py P (purple flowers, dominant) is expressed, p (white flowers, recessive) is hidden

Walter Sutton’s (1903)Chromosome Theory of Heredity Sutton (American) noticed that

chromosomes behaved like Mendel’s factors

Sutton’s Chromosome Theory of Heredity states that the material of inheritance is carried by the genes in chromosomes

Theodor Boveri (German) reached the same conclusion independently

Predicting the Phenotype of Offspring:Using a Punnett Square for One Trait

*Monohybrid cross

*

Genotypic Ratio:

Phenotypic Ratio:

YY : Yy : yy 1 : 2 : 1

Yellow : Green 3 : 1

Predicting the Phenotypes of Offspring:Using a Punnett Square for Two Traits

*Dihybrid cross

*

Phenotypic Ratio:

9 : 3 : 3 : 1Round & Yellow : Round & Green : Wrinkled & Yellow : Wrinkled & Green

Inheritance Pattern - Autosomal Recessive Trait

Genotypes?

Autosomal Recessive Traits in Humans(know all traits in bold print) Albinism – skin, hair, eyes lack melanin (pigment), sensitive to

sunlight Tay-Sachs – fat deposits prevent normal development of brain in

children Cystic Fibrosis – lungs damaged from excessive mucous, respiratory

problems Sickle Cell Anemia – red blood cells misshaped, clog capillaries

circulation problems; many phenotypic effects, depending on where in the body the blockage occurs

Click for Albino - pictures

Inheritance Pattern - Autosomal DominantTrait

Genotypes? (could also be DD)

Autosomal Dominant Traits in Humans(know all traits in bold print) Dimples, freckles, widow’s peak, farsightedness, broad lips, polydactyly

(extra fingers/toes) Dominant disorders are less common than recessive disorders because

carriers (heterozygotes) are affected and may die before reproducingEx.: Huntington disease – deterioration of nervous system, especially

brain

Inheritantce PatternAutosomal Dominant (not in notes)

Variable Expression:Some people have milder / more severe symptoms than others --Age of onset and organs affected may vary

Reduced Penetrance:Some people have the dominant gene but do not have any symptomsEx.: autosomal dominant cancer susceptibility

Pedigrees

Not always labeled

Pedigrees & Genetic Counseling

People with PKU (phenylketonuria) lack an enzyme needed to break down the amino acid phenylalanine, which is found in milk and other foods. Accumulation of the amino acid in the body leads to developmental disabilities and death if not treated.

Individual II-4 has PKU

From H. Genetics Lab :Pedigree -- Dominant or Recessive trait?Not always clear…

Trait appears in every generation More likely to be a dominant trait, but cannot be sure.

* Notice that the hybrid

individuals are not identified. *

Pedigree -- Dominant or Recessive trait?Not always obvious…

Trait skips a generation or more More likely to be a recessive trait, but cannot be sure.

* Notice that the hybrid

individuals are not identified. *

When can we tell for sure if the trait is dominant or

recessive?Do the Pedigree Analysis

to find out!

Pedigree A

Parents have the trait, child does not.

The trait is dominant: the “non-trait” phenotype is

recessive.

Xx

xx

Xx

Xx xx

xx

xx

xx

xx

xx

xxxx xx

Xx

Xx Xx

Xx or XX

MONOHYBRID CROSS

Pedigree B

Parents don’t have the trait, child does.

The trait is recessive: the allele is “hidden” in

the parents.

xx xx

xx (all)

xx

xx

MONOHYBRID CROSS

XX or Xx

Xx Xx

All Xx

XX or Xx XX or Xx

XX or Xx

XX or XxXX or Xx