GREGOR MENDEL

• Austrian monk• Studied at the University of Vienna• Discovered the basic principles of heredity• Worked with breeding garden peas

– Self pollinating– Perfect flowers– Artificially cross pollinated

Mendel’s Pea Plants• True breeding – all

offspring same variety• Hybridization – crossing 2

contrasting true breeding varieties

• P generation – parental generation (true breeding)

• F1 – first filial (hybrids)

• F2 – second filial (from self pollinating F1s)

Mendel tracked heritable characters for 3 generations

MENDEL’S LAWS1. Alternative versions of genes (alleles) account

for variations in inherited characters.2. For each character, an organism inherits two

alleles, one from each parent.3. If the 2 alleles differ, then one, the dominant

allele is fully expressed in the organism’s appearance; the recessive allele has no noticeable effect on the organism’s appearance

4. The 2 alleles for each character segregate during gamete production.

Law of Segregation

• Letters represent alleles (upper case for dominant alleles & lower case for recessive alleles)

• P generation – true breeding plants, matching alleles (PP or pp)

• Gametes contain only one allele

• Fusion of gametes → hybrid F1s

• When hybrids produce gametes, the 2 alleles segregate

• Half the gametes receive the P allele and half receive the p allele

• Punnett squares show possible combinations of alleles in gametes

• Each square is a possible offspring

VOCABULARY

TEST CROSS

DIHYBRID CROSSESGiven: T- tall R - round

t – dwarf r - wrinkled

PROBABILITY LAWS

1. CHANCE HAS NO MEMORY

2. RULE OF MULTIPLICATION: the chance of 2 independent events occurring together is the product of their individual probabilities.

3. RULE OF ADDITION: the probability of an event that can occur in 2 or more different ways is the sum of the separate probabilities.

PROBABILITY PRACTICE

1) If a coin is tossed 7 times and lands heads all seven times, what is the chance of getting heads again?

2) A couple has 3 girls; what is the chance that their fourth child will be another girl?

3) In a deck of 52 cards, what is the chance of drawing:

a) Any red card?b) Any ace?c) Any heart?

PROBABILITY PRACTICE

1. From a normal deck of cards, what is the chance of drawing the Jack of Hearts?

2. When tossing 2 dice, what is the probability of getting a “6” up on both?

3. What is the probability of 2 parents, with genotypes AabbCc x AaBBCc, having a child with the genotype AaBbCc?

AaBbRr x Aabbrr

• What fraction of the offspring will have the following genotypes?– aabbrr - AaBbRr - aaBbrr

• What fraction of the offspring will have at least two recessive phenotypes?

a) list all possible genotypes

b) calculate probabilities (rule of multiplication)

c) pool probabilities (rule of addition)

AaBbRr x Aabbrr

Predict the gametes

• Formula 2n (n = # of heterozygous pairs)

• AA → 20 →1 gamete type (A)

• Aa → 21 → 2 gametes (A or a)

• AABb → 21 →2 gametes (AB or Ab)

• AaBb → 22 → 4 gametes (AB, Ab, aB, ab)

• AaBbDd → 23 → 8 gametes

• AaBbDdFf → 24 →16 gametes

AaBbDd AaBbDdFf

Incomplete Dominance• Alleles for red and

white, neither is dominant.

• Hybrids are a blend of the two alleles and are phenotypically pink

• There is NO allele for pink, therefore NO true breeding pink flowering plants.

• Codominance: both alleles equally expressed. Human blood type, cow coloring

MULTIPLE ALLELES

EPISTASIS • Gene at one locus alters the phenotypic expression of a gene at a second locus

• Gene for fur color: (B) black (b) brown

• 2nd gene deposition of pigment: (C) color (c) white

Polygenic Traits• Skin color and

height in humans• Additive effect of

2 or more genes on 1 phenotype

• Quantitative characters – variation along a continuum

• Dots represent “units” of darkness

NATURE and NURTURE

The product of a genotype is a range of phenotypic possibilities over which there may be variation due to environmental influence. Norm of reaction

- Hydrangea flower color and pH (blue-pink)- Human blood typing: little range (genotype

mandates phenotype.- Human blood counts: vary with altitude,

fitness, infectious agentsNorms of reaction broadest for polygenic traits

like skin color. Multifactorial characters: both genetic & environmental influences.

Mendelian Inheritance in Humans

• Recessive Disorders– Cystic fibrosis - PKU– Tay-Sachs disease– Sickle-cell disease

• Dominant Disorders– Achondroplasia -Polydactyly– Huntington’s disease

• Multifactorial Disorders

SICKLE CELL DISEASE

• Pleiotrophy – one disorder, multiple effects

• Incomplete dominance

• Heterozygotes – sickle cell trait, advantage (resistant to malaria)

PEDIGREES

INHERITANCE PATTERN?

FETAL TESTING

THE CHROMOSOMAL

BASIS OFINHERITANCE

The Chromosomal Basis of Mendel’s Laws

Segregation

R & r alleles Segregate

Only one long chromosomeIn each gamete

Fertilization recombines the R & r alleles

IndependentAssortment

Long and short chromosomes;Arranged in 2 equally likely ways

They assort independently

Fertilization 9:3:3:1 ratio

THOMAS HUNT MORGAN

• Worked with Drosophila• Wild type – phenotype most common in

the wild. Red eyes, gray, normal wings• Mutant phenotypes – white eyes, ebony,

vestigial wings• Discovered sex linkage• Sex linked genes – on sex chromosomes

Sex Linked Inheritance

• Sex linked genes: on the X or Y chromosome

• X & Y NOT homologous• X-linked genes: males

being XY have only one copy/allele, females XX have two copies/alleles

• X-Linked recessive – more common in males

• X-Linked dominant – more common in females

SEX DETERMINATION

• Humans – sex determined by presence or absence of Y chromosome; XX-female; XY-male

• Fruit flies – # of X chromosomes; XX-female; XY-male; XXY-female

• Birds – females are heterogametic

Transmission of Sex-Linked Recessive Traits

Father w/ trait Carrier passes Carrier w/ afflicted

transmits to all trait to ½ sons male; 50% of

daughters ½ daughters children afflicted

Sons afflicted

LINKAGE

• Law of Linear Order: genes on the same chromosome are linked and are inherited in a block.

• Drosophila – 2 linked genes; inherited together– body color (G gray, g ebony) – wing size (L long, l vestigial)

• Test cross by Morgan produced unexpected results

CROSSING OVER

MAPPING

• Linkage map – genetic map based on recombination frequencies

• Map units – one map unit = 1% recombination frequency

• Cytological maps – locate genes with respect to chromosomal features like stained bands

Recombination frequencies: used to construct genetic map

The probability of a crossover between 2 loci is proportional to the distance separating the loci

Sex linked Recessive Disorders in Humans

• Duchenne Muscular Dystrophy

• Hemophilia

• Red/Green Color blindness

X Inactivation

NONDISJUNCTIONleads to aneuploidy (trisomy, monosomy, polyploidy)

CHROMOSOMAL MUTATIONS

Human Chromosomal Disorders

• Down Syndrome (Trisomy 21)

• Turner Syndrome (XO, monosomy X)

• Klinefelter syndrome (XXY)

• XYY, XXX

• Cri du chat (cry of the cat) deletion in #5

• CML (chronic myeloid leukemia) translocation of # 22

• Down Syndrome due to translocation of #21

Genomic Imprinting• Prader-Willi Syndrome:

– Mental retardation, obesity, short stature, small hands & feet (father)

• Angelman Syndrome:– Spontaneous laughter, jerky

movements, other motor and mental symptoms (mother)

• Same cause – partial deletion of chromosome #15

• Genomic Imprinting – gene on one chromosome silenced