Chapter 15 Chromosomal Basis of

Inheritance

Chromosome Theory• Chromosome theory of

inheritance – states that genes have specific loci on the chromosome and they are what go through segregation and independent assortment

• Thomas Hunt Morgan is credited with providing support to theory with work on fruit flies (Drosophila Melanogaster)

- showed early framework for sex linked genes

Sex-linked Genes

• Chromosomal basis of sex determination is simply X or Y.

- one allele from each parent - XX is female -XY is male- ovaries produce only X chromosomes , whereas sperm produce X and Y that

segregate during meiosis

Figure 15.6

Parents

(a) The X-Y system

(b) The X-0 system

(c) The Z-W system

(d) The haplo-diploid system

or

Zygotes (offspring)

or

Sperm Egg

44 +XY

44 +XY

44 +XX

44 +XX

22 +X

22 +X

22 +X

22 +XX

22 + Y

76 +ZW

76 +ZZ

32(Diploid)

16(Haploid)

Sex-linked Genes• Sex linked gene- gene located on the sex chromosome

- historically referred to gene on X chromosome- fathers may pass sex linked allele to daughters but not sons- mothers could pass alleles to both- males have only one locus meaning if they receive recessive allele from the mother, they express the trait

*leads to male more prominently expressing sex-linked recessive disorders

Ex: Muscular dystrophy – lack of muscle protein dystrophin located on the X chromosomeHemophilia- absence of one or more proteins for clotting blood

X Inactivation in Females• Barr body – inactive X chromosome in a cell of a female

-genes will not be expressed-occurs randomly and independently in each

embryonic cell-creates 2 cell types: active X from father and

active X from mother-once inactivated all mitotic descendants have

inactive X• possible cause comes from multiple copies of RNA

transcript gene on Barr body called X-inactive specific transcript (XIST)

Gene Linkage

• Linked genes – genes located on the same chromosome that are inherited together in genetic crosses

• Genetic recombination – production of offspring with combinations of traits different from either parent

*For unlinked genes recombination is caused by random orientation of homologous chromosomes – gives 50% frequency

*Crossing over of homologous chromosomes accounts for the break in connection between genes on the same chromosome and allows for the recombination of linked genes

Gene Maps

• Genetic map – ordered list of genetic loci along a chromosome

• Linkage map – genetic map based on recombination frequencies

map unit – distance between genes; 1 unit = 1% recombination frequency

• Alfred Sturtevant contributed the idea that the farther apart two genes are, the higher the probability of crossover and thus higher frequency of recombination

-possible for genes on the same chromosome to be physically linked but not genetically linked

Abnormal Chromosome Number

• Sex linked traits and mutations are not the only ways of changing phenotypes

• Physical and chemical disturbances, errors in meiosis, or damage to chromosome can alter genes and lead to disorders

• Nondisjunction – condition in which members of a pair of homologous chromosomes do not move apart in meiosis I, or sister chromatids fail to separate in meiosis II

-leads to abnormal chromosome numbers

Abnormal Chromosome Number

• Aneuploidy – condition in which offspring have an abnormal number of a particular chromosome

-trisomic – 3 chromosomes present or 1 extra

-monosomic – 1 chromosome present or 1 missing

• If nondisjunction occurs during mitosis and early in embryonic development then the condition can be passed to a large a number of cells and have a greater effect on organism

Abnormal Sets

• Polyploidy- term for alteration of chromosome sets or multiple chromosome sets

- refers to somatic cells after mitosis-triploidy – 3 sets of chromosomes-tetraploidy – 4 sets

-condition more common in plantsEx: wheat – hexaploidstrawberries - octoploid

Alterations of Structure

Human Disorders• Aneuploid gametes can result

from nondisjunction in meiosis creating aneuploid zygote

-some conditions can cause embryos to spontaneously abort -some survive birth and beyond but have set of symptoms labeled a syndromeEx: Down syndrome or trisomy 21

Aneuploidy of Sex Chromosomes

• Klinefelter syndrome- males born with extra X chromosome (XXY)

-occurs 1 in 2,000 births (2004)- occurs 1 in 500 – 1,000 male births (2014)-presence of sex organs but sterile individual-some breast enlargement or female characterstics

may occur-normal intelligence

-Males with extra Y (XYY) exhibit taller stature than average but not known

syndrome ( 1 in 1,000)

Aneuploidy of Sex Chromosomes

• Females with extra X chromosome (XXX) are normal and healthy

• Turner’s syndrome – (X0) only known monosomy in humans

- 1 in 2,500 female births-individuals appear female but sex organs do not mature and become sterile- normal intelligence

Altered Chromosome Disorders

• Cri du chat – deletion in chromosome 5 that results in mental retardation, small head, facial abnormalities – cat sound cries

-most die as infants or early childhood• Chronic myelogenous leukemia – results from

translocation of chromosomes- results from long portion of chromosome 22 and tip

of chromosome 9 exchange(forms Philadelphia chromosome)

- gene activated causes uncontrolled cell cycle progression

Inheritance patterns

• Genomic imprinting - patterns of methylation passes from parent to offspring keeping record of what occurred during development

-permanently regulates expression of either maternal or paternal allele of certain genes

• Extranuclear genes – found in organelles such as mitochondria and chloroplasts contain small circular DNA that can code for proteins and RNA