Mendelian GeneticsMendelian Genetics

Bio 156

Human Biology for Allied HealthPima CC East

Fall 2006

Gregor MendelGregor Mendel

• In 1865, Mendel published the paper "Experiments in Plant Hybridization"

• studied seven basic characteristics of the pea pod plants

• Developed Mendel’s Laws1882

Mendel’s lawsMendel’s laws

• The sex cell of a plant or animal can only contain one factor (allele) for a specific trait

• Characteristics are inherited independently from other characteristics

• Each inherited characteristic is determined by two heredity factors/genes, – one from each parent– determines whether a gene will be dominant or

recessive.

ChromosomesChromosomes

• Normal Human cells have 46 chromosomes

– 23 homologous chromosome pairs

• 22 Pair are Autosomes–Look similar

• 1 pair are Sex Chromosomes–X and Y

– Females have 2 X chromosomes (XX)

– Males have one X and one Y (XY)

How Many Chromosomes???How Many Chromosomes???

• Pea Plant =14

• Cat = 38

• Dog = 78• Ichthyomys pittieri (semiaquatic rodent)= 94

[highest for a mammal]

• Ophioglossum reticulatum (fern) =1200 or 1260 [highest plant]

• Aulacantha (protozoa) = 1600

(Diploid number)

Parental ChromosomesParental Chromosomes

• 23 chromosomes inherited from mother (ovum)– 22 autosomes & one sex chromosome (X)

• 23 chromosomes inherited from father (sperm)– 22 autosomes & one sex chromosome (X or Y)

• Sperm and egg cells each contain 23 haploid chromosomes– paternal chromosomes combine with maternal

chromosomes

ChromosomesChromosomes

• The 46 human chromosomes – 22 pairs of autosomal chromosomes– 2 sex chromosomes

• almost 3 billion base pairs of DNA

• Chromosomes range in length from about 50 million to 250 million base pairs

• The human genome has an estimated 20,000 to 25,000 genes.

Chromosome 1Chromosome 1

• Chromosome 1 contains nearly twice as many genes as the average chromosome and makes up eight percent of the human genetic code.

• It is packed with 3,141 genes and linked to 350 illnesses including cancer, Alzheimer's and Parkinson's disease

• The sequence of chromosome 1, which is published online by the journal Nature, took a team of 150 British and American scientists 10 years to complete.

Chromosome 1& Human GenomeChromosome 1& Human Genome

• The sequencing of chromosome 1 has led to the identification of more than 1,000 new genes.

• The scientists also identified 4,500 new SNPs -- single nucleotide polymorphisms -- which are the variations in human DNA that make people unique.

Human TraitsHuman Traits

• Researchers have identified over 15,000 genetically inherited human traits. 

• More than 5,000 of them are diseases or other abnormalities. 

Chromosomes and HeredityChromosomes and Heredity

• Heredity = transmission of genetic characteristics from parent to offspring

• Karyotype = chart of chromosomes at metaphase – Chromosomes are arranged in order by size and

structure

Karyotype ProcedureKaryotype Procedure

• involves blocking cells in mitosis• staining the condensed chromosomes with

Giemsa dye. • The dye stains regions of chromosomes that are

rich in the base pairs Adenine (A) and Thymine (T) producing a dark band.

• A common misconception is that bands represent single genes, but in fact the thinnest bands contain over a million base pairs and potentially hundreds of genes

SEM of ChromosomesSEM of Chromosomes

Normal Human Male KaryotypeNormal Human Male Karyotype

Genes and AllelesGenes and Alleles

• Each chromosome carries multiple genes

• Locus (pl. Loci)– location of a gene on chromosome

• Alleles – different forms of gene

• Produce alternate forms of the same trait

– at same locus on the 2 homologous chromosomes

Genes and AllelesGenes and Alleles• Dominant allele

– produces protein responsible for visible trait• If dominant allele present gene expressed• Often written as Upper Case letter (ie. D)

• Recessive allele– Only expressed when no dominant allele

is present• e.g. when both alleles are recessive• Often written as corresponding lower case

letter (ie.d)

Genes and AllelesGenes and Alleles

• Homozygous

– Both homologous chromosomes have same allele for the gene (trait)• Both Dominant: DD• Both Recessive: dd

• Heterozygous

– Homologous chromosomes have different alleles for the a gene• One Dominant & one recessive: Dd

Genes and AllelesGenes and Alleles

• Genotype

– The alleles an individual has for a particular trait• DD or Dd or dd

• Phenotype

– The physical expression of the alleles for a particular trait• What you see…

Genetics of EarlobesGenetics of Earlobes

How to complete a Punnet SquareHow to complete a Punnet Square

1. Determine Genotype of each parent1. AA or Aa or aa

2. Write Down the cross1. AA x aa

3. Draw a Punnet Square

How to complete a Punnet SquareHow to complete a Punnet SquareAA x aaAA x aa

4. Split the letters of each parent genotype and put them outside the Punnet Square

A A

a

a

How to complete a Punnet SquareHow to complete a Punnet Square

5. Determine the offspring by Filling in Punnet square

A A

a A

a A

How to complete a Punnet SquareHow to complete a Punnet Square

5. Determine the offspring by Filling in Punnet square

A A

a A A

a A A

How to complete a Punnet SquareHow to complete a Punnet Square

5. Determine the offspring by Filling in Punnet square

A A

a Aa Aa

a A A

How to complete a Punnet SquareHow to complete a Punnet Square

5. Determine the offspring by Filling in Punnet square

A A

a Aa Aa

a Aa Aa

How to complete a Punnet SquareHow to complete a Punnet Square

5. Determine the offspring by Filling in Punnet square

A A

a Aa Aa

a Aa Aa

Parents

AA x aa

Offspring

100% Aa

Genetics of EarlobesGenetics of Earlobes

Genetics of EarlobesGenetics of Earlobes

Punnett square

d

D

D d

Parents

Dd x Dd

D

D

d

D

D d

Parents

Dd x Dd

D

D

d

d

d

D

D d

Parents

Dd x Dd

D

D

d

d

D D

d

D

D d

Parents

Dd x Dd

D

D

d

d

D D

d d

d

D

D d

Parents

Dd x Dd Genotype

25% DD

50% Dd

25% dd

Phenotype

75% det

25% att

Sex-Linked InheritanceSex-Linked Inheritance

• Sex Linked Traits – Genes that are carried by either sex chromosome– X chromosome contains about 1000

human X-linked genes– Smaller Y Chromosome only ~78

genes

• Men only have one X chromosome– No corresponding gene on Y so gene

on X expressed (even if recessive)

Sex-Linked InheritanceSex-Linked Inheritance

• Recessive allele on X, no gene locus for trait on Y• so hemophilia more common in men

– (mother must be carrier)

HemizygousHemizygous

• An individual who has only one member of a chromosome pair or chromosome segment rather than the usual two

• X-linked genes in males who under usual circumstances have only one X chromosome

Gene PoolGene Pool

• Gene pool– collective genetic makeup of whole

population– the complete set of unique alleles that would be

found by inspecting the genetic material of every living member of that species or population.

Alleles at the Population LevelAlleles at the Population Level

• Dominance and recessiveness of allele do not determine frequency in a population

• Some recessive alleles are the most common – blood type O

• Some dominant alleles are rare– polydactyly and blood type AB,

ABO grouping Percentages*ABO grouping Percentages*

O + 38% O- 7%

A+ 34% A- 6%

B+ 9% B- 2%

AB+ 3% AB- 1%

*In US population

Country U.S. G.B. Germany Japan China

ABO Type % % % % %

O 46 47 41 30 35

A 40 42 45 38 27

B 10 8 10 22 26

AB 4 3 4 10 12

Percentage of each ABO Blood Type by Country

Some exceptions to Some exceptions to Simple Mendelian GeneticsSimple Mendelian Genetics

(we will only discuss a few…)

Multiple AllelesMultiple Alleles

• Multiple alleles– more than 2 alleles for a trait

• ABO blood type system – three alleles (A, B, and O)

• each individual only inherits two of them• one from each parent

Multiple AllelesMultiple Alleles

• human Human Leukocyte Antigen (HLA) system– responsible for identifying and rejecting foreign

tissue in our bodies– can have at least 30,000,000 different

genotypes. 

• It is the HLA system which causes the rejection of organ transplants.

CodominanceCodominance

• Codominant Allele– both alleles expressed, IAIB = type AB

blood

Incomplete DominanceIncomplete Dominance

• Incomplete dominance– phenotype intermediate between traits for

each allele

Polygenic InheritancePolygenic Inheritance

• Polygenic Inheritance – 2 or more genes combine their effects to

produce single phenotypic trait• skin and eye color, alcoholism and heart disease

PleiotropyPleiotropy• Single gene causes multiple phenotypic

traits (ex. sickle-cell disease)– sticky, fragile, abnormal shaped red blood

cells at low oxygen levels cause anemia and enlarged spleen

Penetrance and Environmental EffectsPenetrance and Environmental Effects

• Penetrance– % of population to

express predicted phenotype given their genotypes

• Role of environment– brown eye color

requires phenylalanine from diet to produce melanin, the eye pigment

Eye Color…Eye Color…

• Eye color comes from a combination of two black and yellow pigments called melanin in the iris of your eye

• no melanin in the front part of your iris, you have blue eyes.

• An increasing proportion of the yellow melanin, in combination with the black melanin, results in shades of colors between brown and blue, including green and hazel.

Eye Color…Eye Color…

• Three gene pairs controlling human eye color are known.

• Two of the gene pairs occur on chromosome pair 15 – The bey 2 gene (EYCL3),

• has a brown and a blue allele

– The bey 1• is a central brown eye color gene

• One occurs on chromosome pair 19.– The gey gene (EYCL1)

• has a green and a blue allele..

Modeling Eye Color…Modeling Eye Color…

• The bey 2 gene (EYCL3), • brown allele Dominant• blue allele recessive

• The gey gene (EYCL1)• Green allele Dominant to blue BUT recessive to

Brown• blue allele recessive

• And this still doesn’t explain, hazel, grey and other colors… MORE TO DISCOVER!!!