InheritanceInheritance

Chapter 29Chapter 29

Gregor Gregor MendalMendal

“Father of Genetics”

1822 - 1884

What Mendal didWhat Mendal did He bred peas in the monastery garden at Brno, He bred peas in the monastery garden at Brno,

Czech Republic (then part of the Czech Republic (then part of the AustroHungarian Empire).AustroHungarian Empire).

Observed occasional variations in the Observed occasional variations in the appearance of these plants.appearance of these plants.

Selectively bred plants to consistently produce Selectively bred plants to consistently produce “characteristics” that were unusual.“characteristics” that were unusual.

Saw a pattern in the way that the unusual Saw a pattern in the way that the unusual characteristics showed up.characteristics showed up.

Was the first to propose that these Was the first to propose that these characteristics were passed from one characteristics were passed from one generation to another by the gametes.generation to another by the gametes.

The Abby where Mendal workedThe Abby where Mendal worked

What Mendal did not doWhat Mendal did not do

He didn’t use the word “gene” to refer to He didn’t use the word “gene” to refer to subject of his work.subject of his work.

He didn’t see chromosomes.He didn’t see chromosomes. He never used a Punnett square.He never used a Punnett square. He never achieved fame in his lifetime for He never achieved fame in his lifetime for

his work.his work.

Charles DarwinCharles Darwin1809 - 1882

•Proposed the “Theory of Evolution”.• Actually, talked about “descent with modification from a common ancestor”. He didn’t use the word “evolution” very often.• Voyage of the Beagle 1831 – 1836.• Presented paper with Alfred Russell Wallace in 1858.• Published first edition of “Origin of Species” in 1859.

Some VocabularySome Vocabulary

Genetics – study of inheritance.Genetics – study of inheritance. Autosomes – the 22 pairs of Autosomes – the 22 pairs of

chromosomes that do not determine chromosomes that do not determine genetic sex.genetic sex.

Sex chromosomes – the 23Sex chromosomes – the 23rdrd pair, the X pair, the X and the Y.and the Y.

Karyotype – the diploid chromosomes Karyotype – the diploid chromosomes displayed in their condensed form and displayed in their condensed form and paired as homologspaired as homologs

A typical karyotypeA typical karyotype

More VocabularyMore Vocabulary Alleles - a matched pair of two genes, Alleles - a matched pair of two genes,

coding for the same or alternative forms of coding for the same or alternative forms of a particular trait. Found at the same a particular trait. Found at the same location (location (locuslocus) on homologous ) on homologous chromosomes.chromosomes.

Homozygous – having the same alleles for Homozygous – having the same alleles for a traita trait

Heterozygous – having different alleles for Heterozygous – having different alleles for the same trait.the same trait.

More wordsMore words Dominant – an allele that expresses itself and Dominant – an allele that expresses itself and

masks its partner. Example: brown hair is masks its partner. Example: brown hair is dominant over blond.dominant over blond.

Recessive – the reverse of the above. The Recessive – the reverse of the above. The allele that is maskedallele that is masked

Allele pairs are expressed as a pair of letters Allele pairs are expressed as a pair of letters representing the trait. Example: Mendal’s peas representing the trait. Example: Mendal’s peas came in tall and short. Tall is the dominant came in tall and short. Tall is the dominant allele for height in peas. Therefore it is written allele for height in peas. Therefore it is written as a capital “T”.as a capital “T”.

A heterozyote for height would be Tt, with the A heterozyote for height would be Tt, with the lowercase t representing the recessive.lowercase t representing the recessive.

Genotype vs. PhenotypeGenotype vs. Phenotype Genotype – the actual alleles an organism has Genotype – the actual alleles an organism has

is it’s is it’s genotypegenotype. In our heterozygote pea plant . In our heterozygote pea plant that would be Tt.that would be Tt.

Phenotype – that which is expressed. Our pea Phenotype – that which is expressed. Our pea plant maybe genotypically heterozygotic but plant maybe genotypically heterozygotic but phenotypically it is tall.phenotypically it is tall.

Homozygote dominant = TT phenotype = tallHomozygote dominant = TT phenotype = tall Homozygote recessive = tt phenotype = shortHomozygote recessive = tt phenotype = short HeterozygoteHeterozygote = Tt phenotype = tall= Tt phenotype = tall

Sources of Sources of variation: variation:

segregation segregation & &

independent independent assortmentassortmentAssortment leads to many possibilities as far as gamete formation goes.For any genome it can be calculated as 2n, where n=the number of chromosome pairs.

So for a human with 23 So for a human with 23 chromosome pairs, the possible chromosome pairs, the possible combinations of gametes = 2combinations of gametes = 22323

oror8,388,608!8,388,608!

(and that’s with out recombination)(and that’s with out recombination)

Mendal’s LawsMendal’s Laws

Mendal discovered that if you bred plants Mendal discovered that if you bred plants that had two alleles for each trait that you that had two alleles for each trait that you would get the same ratios of phenotypes & would get the same ratios of phenotypes & genotypes whenever you crossed genotypes whenever you crossed heterozygotes. It was like clockwork!heterozygotes. It was like clockwork!

This was because of independent This was because of independent assortment and segregation, which assortment and segregation, which became known as “Mendal’s Laws”became known as “Mendal’s Laws”

It works like this…It works like this…

Phenotypic ratio = 3:1 or 3 tall : 1 short

Genotypic ratio = 1:2:1 or 1 homozygote dominant 2 heterozygotes

1 homozygote recessive

Example: PKUExample: PKU

Violation of Mendal’s LawsViolation of Mendal’s Laws

Mendal’s laws only hold if: Mendal’s laws only hold if: there is random fertilization there is random fertilization the alleles are located on separate the alleles are located on separate

chromosomeschromosomes the alleles have a simple dominant/recessive the alleles have a simple dominant/recessive

relationshiprelationship there are only two alleles for that traitthere are only two alleles for that trait they are not lethal to the zygotethey are not lethal to the zygote

Recombination interferes with Recombination interferes with Mendal’s lawsMendal’s laws

Types of inheritanceTypes of inheritanceAside from simple dominant/recessiveAside from simple dominant/recessive

Incomplete dominance – a dominant allele does Incomplete dominance – a dominant allele does not completely mask the recessive (red flower + not completely mask the recessive (red flower + white flower = pink flower).white flower = pink flower).

Codominance – both traits are expressed Codominance – both traits are expressed together (red flower + white flower = stripes).together (red flower + white flower = stripes).

Multiple alleles – More than one allele for a trait. Multiple alleles – More than one allele for a trait. ABO blood group is an example.ABO blood group is an example.

Polygene – several alleles interact to produce a Polygene – several alleles interact to produce a trait. Results are a trait. Results are a continuous continuous or or quantitativequantitative phenotype, as in skin color.phenotype, as in skin color.

Incomplete Incomplete dominance:dominance:Sickle CellSickle Cell

Codominance of multiple allelesCodominance of multiple alleles

Polygenic inheritancePolygenic inheritance

Sex-linked inheritanceSex-linked inheritance Males only have one X chromosome. Males only have one X chromosome.

Therefore, if a trait is found only on the X it will Therefore, if a trait is found only on the X it will be expressed in a male regardless of whether it be expressed in a male regardless of whether it is dominant or recessive.is dominant or recessive.

X – inactivation occurs in females. Every X – inactivation occurs in females. Every normal woman has two Xs but they only need normal woman has two Xs but they only need one. Therefore, one X chromosome turns off, one. Therefore, one X chromosome turns off, forming a Barr body.forming a Barr body.

Because X – inactivation is random in most Because X – inactivation is random in most cases, it leads to a cases, it leads to a fine mosaicfine mosaic of cells in of cells in females.females.

22 autosomes & 1 set of sex chromosomes22 autosomes & 1 set of sex chromosomes

Sex Sex determination determination

in humansin humans

Color-Color-blindness:blindness:

a sex-linked a sex-linked traittrait

Environmental influencesEnvironmental influences

Phenocopy – Developmental influences Phenocopy – Developmental influences impact genetic expression in ways that impact genetic expression in ways that appear to be genetic but are not appear to be genetic but are not inheritable.inheritable.

Temperature, nutrition, non-genetic Temperature, nutrition, non-genetic pathologies can have impacts that are pathologies can have impacts that are expressed in ways that appear genetic.expressed in ways that appear genetic.

Genetic defectsGenetic defects Aneuploidy – a defective set of genes.Aneuploidy – a defective set of genes. Triploidy – an extra set of chromosomesTriploidy – an extra set of chromosomes Trisomy – an extra single chromosomeTrisomy – an extra single chromosome Monosomy – a missing homologMonosomy – a missing homolog

Trisomy of the 23Trisomy of the 23rdrd chromosome – chromosome –

XXX = “super female”XXX = “super female”

XXY = Klinefelter’s syndromeXXY = Klinefelter’s syndrome Trisomy of the 21Trisomy of the 21stst chromosome leads to chromosome leads to

Down’s SyndromeDown’s Syndrome..

Down’s syndromeDown’s syndrome

Klinefelter’sKlinefelter’s - -a type trisomy a type trisomy affecting the affecting the

sex sex chromosomeschromosomes

Turner’s Syndrome: monosomy of Turner’s Syndrome: monosomy of the 23the 23rdrd chromosome, X_ chromosome, X_

Monosomy of the 23rd chromosomeMonosomy of the 23rd chromosome

Name that condition!

A A Pedigree:Pedigree:tracking tracking genetic genetic traitstraits

A Peek A Peek into the into the Future:Future:

Screening Screening for genetic for genetic disordersdisorders

That’s all folks!That’s all folks!