Chapter 11Genetics

GeneticsAll living things have a set of

characteristics inherited from its parent or parents

Genetics – the study of heredityTrait – a specific characteristic, such as

seed colorGenes – chemical factors (DNA

segments) that control traits. Usually code for proteins

Alleles – forms of a gene, like T (tall) or t (short)

Gregor MendelMendel was a monk in charge of the

garden monastery◦Mendel had stocks of plants that

produced only tall, only short, only green seeds, and only yellow seeds

◦He studies cross-bred plants with different characteristics and studied the results

◦The offspring (F1) of crosses between parents (P) with different traits are called hybrids

Gregor Mendel

Gregor MendelAll of the F1 generation had

characteristic of only one parent (no blending)◦Each characteristic was controlled by

1 gene with two contrasting forms, called alleles

◦Principle of dominance states that some alleles are dominant while others are recessive

Gregor Mendel

Gregor MendelHe then crossed F1 plants to produce

F2 plants◦Recessive traits reappeared, roughly

¼ had the recessive traits◦The two alleles of the F1 plant

segregate from each other so that a gamete only has 1 copy of the gene

◦Each F1 plant gamete has either the dominant or recessive (tall or short)

Gregor Mendel

MendelGregor Mendel concluded:

◦Inheritance is controlled by factors passed from one generation to the next.

◦Some alleles are dominant and others are recessive.

◦Segregation - the alleles (separate) during the formation of gametes.

◦Independent assortment - genes for different traits can segregate independently during the formation of gametes.

Punnett SquaresPunnett Squares – tool used to

predict the probability of offspring with a certain trait or genotype.

Monohybrid cross – Punnett square with one trait, 4 squares.

Genetic TermsDominant – occurs in the phenotype if

one allele is present, TRecessive – must be homozygous to

occur in the phenotypeHomozygous – having two identical

alleles for a trait, TT or ttHeterozygous – having two different

alleles for a trait, TtPhenotype – physical characteristic,

tallGenotype – genetic makeup, TT or Tt

Monohybrid Cross

Dihybrid CrossSince alleles can segregate

independently, the inheritance of one trait doesn’t affect the inheritance of another (seed color and seed shape)

A dihybrid cross involves crossing two different traits of the F1 generation (both parents are heterozygous for both traits)

Dihybrid Cross

Complex Forms of InheritanceSome alleles are neither dominant or

recessive, and many traits are controlled by multiple alleles or multiple genes

Codominance – both alleles are present in the phenotype, the black and white chicken, blood types

 Incomplete dominance – the alleles

blend to produce the phenotype, red + white = pink

Blood Types: Codominance

Incomplete dominance

Complex Forms of InheritanceMultiple alleles – more than 2 alleles

are possible choices, although only 2 can be used at a time, ex. ABO blood groups, rabbit coat color

Polygenic traits – controlled by 2 or more genes, ex. skin color

Sometimes expression is also affected by the environment; height and flower color of a sunflower are affected by genes and climate, soil, and water availability

Multiple Alleles

Sex-linkedGenes that are located on sex

chromosomes are said to be sex-linked genes◦Since males only have 1 X

chromosome, all X-linked alleles are expressed, even if recessive

◦For females, a recessive allele must be with another recessive to be expressed

◦Colorblindness and hemophilia is an example

Sex-linked

Chapter 14Heredity

Human HeredityA karyotype is a picture of

chromosomes arranged in their pairs (46, 23 pairs in humans)

Male Female

Human HeredityAll egg cells carry an X chromosome,

half sperm cells carry an X, half a Y◦Leads to half combined becoming

either XY, or XXAutosomes are non-sex chromosomes,

labeled pairs 1-22 chromosomesSex chromosomes are the 23rd pair of

chromosomes

Human HeredityTo study how traits are passed from

generation to generation a pedigree chart is used◦Males are squares◦Females are circles◦Shaded circle/squares have the trait◦Horizontal lines represent marriage◦Vertical lines connect parents to their

offspringWhite forelock (lock of hair just above

the forehead) is dominant

Human Heredity

Human HeredityMany disorders are caused by an

autosomal recessive allele (disease is only apparent in the homozygous recessive condition)

Some disorders are expressed with only one allele (dominant), which means the disease is apparent in the heterozygous condition

Some disorders are caused by a codominant alleles

Human Heredity

Disorder(s) Inheritance pattern

Symptoms

Cystic fibrosis Simple (autosomal) recessive

Mucus in lungs, digestive tract, liver

Sickle cell disease

Simple (autosomal) recessive (codominant on molecular level)

Sickle shaped red blood cells; joint pain, anemia

Huntington’s disease

Simple (autosomal) dominant

Mental deterioration and uncontrollable movements; onset -35-50

Colorblindness 

Sex-linked recessive Difficulty discerning colors

Hemophilia Sex-linked recessive Blood doesn’t clot correctly, missing Factor VIII

Down syndrome (Trisomy 21)

Trisomy (3 chromosomes instead of 2)

Mild to severe mental retardation

Human HeredityCystic fibrosis leads to serious

digestive problems◦Thick, heavy mucus clogs lungs and

breathing passages◦Since it is recessive, must have 2

alleles to be affectedSickle cell disease leads to

bent/twisted shape of RBCs (can clog in capillaries)◦Heterozygous individuals are

generally healthy and more resistant to malaria

Chromosomal DisordersDown syndrome (trisomy 21)

◦Nondisjunction occurs when homologous chromosomes don’t separate during meiosis-leads to abnormal number of chromosomes

◦3 copies of chromosome 21◦Mild to severe mental disability, and higher

susceptibility to many diseasesSex chromosome nondisjunction

◦Turner syndrome (X): sterile◦Klinefelter’s syndrome (XXY): generally

sterile