Chapter 14~Mendel & The Gene

Idea

Mendelian genetics Character – heritable feature that

varies among individuals: fur color, flower color, height

Trait – variant for a character: brown/white, tall/short

True-bred (self-pollination) – all offspring of same variety

Hybridization (cross-pollination) – crossing of 2 different true-breds

P generation (parents) F1 generation (first filial generation)

– Offspring of P generation F2 generation – offspring of F1

Leading to the Law of Segregation Alternative versions of genes

(alleles) account for variations in inherited characteristics

For each character, an organism inherits 2 alleles, one from each parent

If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance

Mendel’s Law of Segregation– The alleles for each character

segregate (separate) during gamete production (meiosis).

Genetic vocabulary……. Punnett square: predicts the

results of a genetic cross between individuals of known genotype

Homozygous: pair of identical alleles for a character

Heterozygous: two different alleles for a gene

Phenotype: an organism’s traits Genotype: an organism’s genetic

makeup Testcross: breeding of a recessive

homozygote X dominate phenotype (but unknown genotype)

The Law of Independent Assortment Law of Segregation

involves 1 character. What about 2 (or more) characters?

Monohybrid cross (cross of 1 character) vs. dihybrid cross (cross of 2 characters)

Mendel’s Law of Independent Assortment– The two pairs of alleles

segregate independently of each other.

Non-single gene genetics, I Spectrum of Dominance Complere dominance: dominant only

expressed Incomplete dominance: appearance between

the phenotypes of the 2 parents. Ex: snapdragons

Codominance: two alleles affect the phenotype in separate, distinguishable ways.– Ex: Tay-Sachs disease – enzyme to

metabolize lipids in brain doesn’t work: AA – all normal, Aa – ½ normal,

½ dysfunctional, aa – all dysfunctional

Multiple alleles: more than 2 possible alleles for a gene. – Ex: human blood types,

rabbit fur color

Non-single gene genetics, II Pleiotropy: genes with multiple

phenotypic effect. – Ex: sickle-cell anemia, cystic

fibrosis Epistasis: a gene at one locus

(chromosomal location) affects the phenotypic expression of a gene at a second locus. – Ex: mice coat color – 1 gene

determines black/brown, 1 gene determines if there is pigment

Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character – Ex: human skin pigmentation

(3 genes) and height Nature vs Nurture

Human disorders The family pedigree

– Circle – female; Square – male– Shaded – have trait; Not shaded – trait isn’t expressed

Recessive disorders:– Albinism– Cystic fibrosis – increase of concentration in extracellular

chloride causes mucus build up in lungs/digestive tract• Most common• Lethal • 1/2500 of European descent

– Tay-Sachs

Human Disorders cont.– Sickle-cell – amino acid substitution in hemoglobin

protein• Shape causes clumping• 1/400 of African descent• Large # of heterozygous - Malaria

Dominant disorders– Achondroplasia - dwarfism– Huntington’s – degenerative disease of nervous system

• Appears btw 35-45• Fatal

Human Disorders cont. Testing

– Amniocentesis – insert a needle into uterus

• Test on amniotic fluid

• 14th-16th week– Chorionic villus

sampling (CVS) – insert tube through cervix into uterus

• Take tissue from placenta

Chromosomal Linkage Thomas Hunt Morgan – work

proved that genes are on specific chromosomes

Worked with Drosophilia melanogaster (fruit fly)

XX (female) vs. XY (male) Sex-linkage: genes located on a

sex chromosome Linked genes: genes located on

the same chromosome that tend to be inherited together

Human sex-linkage SRY gene: gene on Y chromosome that triggers the development of testes Fathers= pass X-linked alleles to all daughters only (but not to sons) Mothers= pass X-linked alleles to both sons & daughters Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy (MD); hemophilia

X-inactivation: 2nd X chromosome in females condenses into a Barr body (randomly) – genes are only expressed from one

Females may be a mosaic Barr body is reactivated in ovaries Ex. Tortoiseshell cats &

Calico cats (white determined by another gene)

Chromosomal errors, I Nondisjunction: members of a

pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II

Aneuploidy: chromosome number is abnormal– Monosomy~ missing

chromosome–Turner syndrome

– Trisomy ~ extra chromosome - Down syndrome (pair 21), Kleinfelter syndrome

– Polyploidy~ extra sets of chromosomes – common in plants

Chromosomal errors, II Alterations of chromosomal structure:

– Deletion: removal of a chromosomal segment

– Duplication: repeats a chromosomal segment

– Inversion: segment reversal in a chromosome

– Translocation: movement of a chromosomal segment to another