BIOL 111 Chapter 15

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Note: extra credit quiz available on e-Learning

Chromosomal Basis of Inheritance

Sex determination

Sex chromosomes: XY system (in humans and many other animals)

• Grasshoppers use XX (female) vs. X (male)
• Chickens ZW (female) vs. ZZ (male)
• Bees and Wasps diploid (female) vs. haploid (male)

Sex-linked genes:

SRY

Sex-determining Region of Y chromosome

DAX

Dosage sensitive gene on X chromosome

Presence of SRY influences development of male gonads; hormones from gonads specify secondary sexual features

Sex-linked Traits/Disorders

Genes found on X or Y chromosomes (most on X)

Important: Sex-linked traits were one of the original pieces of evidence for genes associating with chromosomes.

Female: Carrier if heterozygous (XX), or affected if homozygous (XX)

Male: hemizygous affected if he has the gene: XY

EX: Color-blindness and hemophilia

Affected Male (XⁿY) × Normal Female (X^NX^N): 100% Females would be carriers, 100% Males would be normal

Morgan

Discovery Timeline
1860	Mendel's laws of inheritance
1870-1890	Physical process of Mitosis/Meiosis discovered
1902	Sutton and Boveri Propose Chromosomal theory of inheritance (note the word theory)

Morgan provided physical evidence for genes associating with chromosome by using Drosophila melanogaster (fruit flies) for crossing and tracing traits

Experimental Process

Eye color:

Wild type/normal/dominant = Red (+)

Mutant/recessive = White (W)

Red female × White male ⇒ all F₁ offspring had red eyes:

	XW	Y
X+	X+XW	X+Y
X+	X+XW	X+Y

Red F₁ × Red F₁ ⇒ all F₂ females had red eyes, 50% of F₂ males had white eyes and 50% had red eyes:

	X+	Y
X+	X+X+	X+Y
XW	X+XW	XWY

Correlation: A trait (eye-color, in this case) correlates with an individual's sex lends support to chromosomal theory of inheritance

Is a white-eyed female possible? Yes:

	XW	Y
X+	X+XW	X+Y
XW	XWXW	XWY

Genetic Variation: Recombination of Genes

Independent assortment recombines unlinked genes

Test Cross Review

Recall pea plant color: YyRr × yyrr ⇒ 50% will look like parent and 50% will look different (recombinant)

Important: If this is the case, then the genes are on different chromosomes (unlinked)

Morgan's Test Cross

b+ b+ vg+ vg+ × b b vg vg ⇒ B B Vg Vg × b b vg vg (wild = dominant)

If on different chromosomes

F₁ B b Vg vg × (test) b b vg vg → 1 B b Vg vg : 1 b b vg vg : 1 B b vg vg : 1 b b Vg vg (50% look like parents, 50% recombinant)

If on same chromosome

F₁ B b Vg vg × (test) b b vg vg → 1 B b Vg bg : 1 b b vg vg : 0 B b vg vg : 0 b b Vg vg (100% look like parents)

Explanation: Crossing over

During creation of gametes, (B Vg)(b vg) on homologous chromosomes crosses to form (B vg)(b Vg). The crossed

Important: If test cross results show >50% look like parent and <50% recombinant, then linked genes tend to be inherited together, but not always due to crossing over

Abnormal Chromosome Number

Nondisjunction

when homologous chromosomes or sister chromatids do not separate

Aneuploidy

offspring with abnormal number of a particular chromosome (monosomy, trisomy) (one less or one more)

EX: Down syndrome -- trisomy 21

EX: Klinefelter syndrome (XXY); phenotypically male, but sterile

EX: Turner syndrome (XO); phenotypically female, but sterile

Polyploidy

offspring with more than 2 complete chromosome sets

EX: triploidy (3N), Tetraploid (4N), etc. in plants (massive hybrid traits)

If nondisjunction occurs in Meiosis I, half of gametes have $n+1$, half have $n-1$

If nondisjunction occurs in Meiosis II, half of gametes are normal, half are malformed ($n+1$ $n-1$, $n$, $n$)

Altered Chromosome Structure

Deletion

removal of sequence

ABCDEFGH → ABCEFGH

Duplication

repeated sequence

ABCDEFGH → ABCBCDEFGH

Inversion

Sequence reversed

ABCDEFGH → ADCBEFGH

Reciprocal translocation

two parts of different chromosomes swapped.

ABCDE + FGHIJ → ABHIE + FGCDJ

Thursday, November 4, 2010

Testing for Genetic Disorders

1. Identifying carriers (in vitro fertilization; screen embryos)
2. Fetal Testing (Amniocentesis; Chorionic villus sampling; Ultrasound)
3. Newborn Screening for PKU -- Treated with special diet

Methylation of Alleles

Genomic Imprinting

"Imprinted" (methylated allele) Deactivate or silence genes

Only in mammals: In gametes, imprints are "erased" and reset according to sex of individual

Organelle DNA

Most organelles are inherited from mom

Mitochondria have their own DNA; maternal lineage