Chapter 12- Human Genetics

Homologous Chromosomes

Autosomes- All other chromosomes except the X and Y chromosome

Generally speaking, homologous chromosomes are alike in length, shape and gene sequence

Sex chromosomes= X and Y chromosomes These are physically different but able to synapse during meiosis

Karyotype

Karyotype= number of metaphase chromosomes and their defining characteristics

Cells are cultured in vitro (in glass) Colchicine is added- blocks formation of

microtubule spindles- arrests cells in metaphase

Karyotype

Karyotypes are pictures of paired human chromosomes

Used in identifying chromosome abnormalities

Sex Determination in Humans

XX= female XY= male X chromosome carries over 300 genes Y chromosome carries “male-

determining gene” . Expression of this gene leads to formationof testes. Absence of this gene- ovaries automatically form

SRY Gene

SRY= Sex-determining region of the Y chromosome

First 4 weeks of its existence, human embryo is neither male or female

SRY gene regulates many proteins that regulate reactions that are necessary for sex determination

Early Questions About Gene Locations Sutton- Chromosome Theory of Inheritance=

Chromosomes are the units of heredity Morgan- genes have specific locations on

specific chromosomes Morgan performed experiments using

Drosophila melanogaster Found evidence of the gene for eye color on

the X chromosome

Characteristics of Drosophila melanogaster

1. Easy to maintain and breed 2. About 0.1 inch long- so tiny that

hundreds can be kept in a jar 3. Have a reproductive cycle of 10-15

days, therefore they can produce many generations of offspring in a matter of weeks

Morgan’s Experiment

In fruit flies- alleles for red eyes (R) are dominant over alleles for white eyes (r).

Morgan hypothesized that the allele for eye color is carried on the X chromosome

P1 generation – Cross a white-eyed male and a homozygous red-eyed female

Sex-linked Traits

Example: red eye color is a dominant trait carried on the X chromosomes in fruit flies; white is recessive

Cross red eyed female with a white eyed male

Get: 2 heterozygous red eyed females and 2 red eyed males

Xr Y

XR XR Xr XRY

XR XR Xr XRY

Sex-linked Traits Now cross the F1: red

eyed male with heterozygous red eyed female

You get:– 1 homozygous red eyed

female– 1 heterozygous red eyed

female– 1 red eyed male– 1 white eyed male

XR Y

XR XR XR XRY

Xr XR Xr XrY

Parental Generation (P1)

F1 generation

F2 generation

Conclusion

The gene for eye color in Drosophila must be carried on the X chromosome

Females have two X chromosomes Males have only one X chromosome so

whatever allele in on the X chromosome for eye color is expressed

Morgan concluded that genes for certain traits are carried on the X chromosomes and that chromosomes and their genes segregate during meiosis

The discovery of sex-linked traits explained why some characteristics caused by recessive genes are appear far more often in males since males have only one X chromosome and females have two X chromosomes

Sex- Linked Trait

1. Sex- linked trait = a trait that is determined by alleles carried only on an X chromosome

2.In humans sex linked traits are found on the X chromosome which is much larger than the Y chromosome

3.In most organisms: Males are XY Females are XX

Inheritance of human traits

Sex-linked Traits - determined by X and Y chromosomes (sex chromosomes)

Recessive traits rarely occur in females because they have two X chromosomes, the dominant gene on one X can mask the recessive gene on the second X

Recessive traits in males are expressed because they only have one X

Inheritance of human traits - examples of sex-linked traits Color vision

– the gene for color vision is located on the X

– the recessive gene results in colorblindness (inability to see certain colors)

– Genotypes: XC XC, XC

Xc, Xc Xc, XCY, and XcY

Hemophilia– a protein needed for

normal blood clotting is located on the X

– the recessive gene results in hemophilia

– individuals with this disease can bleed to death from a tiny cut - their blood does not clot normally

Inheritance of human traits Sex-Influenced Traits - traits that are generally

associated with one sex but is produced by genes carried on autosomes

Example: Baldness– the allele coding for baldness HB is dominant in males and

recessive in females– The allele that coding for normal hair HN is dominant in

females and recessive in males– HN HN - most likely keep hair male and female– HN HB - male will lose hair, female will keep hair– HB HB - most likely lose hair male and female

Linkage Groups

Linkage groups- the genes located on each type of chromosome

Drosophila has 4 linkage groups Humans have 23 linkage groups Crossing over happens among linkage

groups

Probability of a crossover is proportional to the distance between genes

The further apart two genes are, the more likely they will be to cross over

Use patterns of crossing over to map genes on the chromosome

Changes in Chromosome Structure Deletion Inversion Translocation Duplication

Deletions

Loss of a chromosome region May be loss of one or more genes-

almost always causes problems Caused by irradiaiton, viral attck,

chemical action, & other environmetnal factors

Ex: Cat-cry disorder- deletion form chromosome 5

Deletions- cont

Symptoms of cat-cry disorder– abnormally shaped larynx– Infant produces mewing sounds– Mental retardation

Inversion

Segment of DNA detached from chromosome and reattaches in the reverse order

Reversal alters the position and order of the chromosome’s genes

Affects the way the base sequence is read and translated

Translocation

Part of one chromosome exchanges places with the corresponding part of another nonhomologous chromosome

Ex; #8 and #14 translocate- form of cancer results

Duplication

Gene sequences are repeated several to many times

Contained even on normal chromosomes- some DNA duplications are built into the species

EX: hemoglobin in humans and primates- have multiple copies of similar gene sequences

Duplication cont.

18 of 23 pairs of human chromosomes are nearly identical to corresponding chromosome in chimpanzees and gorillas

Ex: fragile X syndrome-abnormally constricted region on X chromosome

Changes in Chromosome Number Aneuploidy= one more or one less

chromosome Causes many miscarriages

Polyploidy= inheritance of three or more of each type of chromosome

Ex: 1/2 flowering plants, some insects, fishes & other animals

Polyploidy is lethal for humans

Chromosome number can change during– mitosis– meiosis– fertilization

Changes in Chromosome # cont.

Nondisjunction= one or more pairs of chromosomes fails to separate properly during mitosis or meiosis

Results in: Trisomy= 3 of one type of chromosome Monosomy= 1 of one type of

chromosome

Changes in Autosome Number

Down’s syndrome= trisomy 21 Syndrome means a set of symptoms

that characterize a disorder Symptoms- mental impairment,

abnormal skeletal development, some have heart defects

Changes in the Number of Sex Chromosomes Turner’s syndrome= XO 75% from nondisjunction in dad 98% of XO zygotes spontaneously abort Symptoms- short, infertile, no functional

ovaries Some benefit form hormone therapy and

corrective surgery

Changes in Sex Chromosome #

Klinefelter Syndrome= XXY 67% from nondisjunction in mom Symptoms develop after puberty- taller,

sterile or low fertility, smaller testes, sparse facial hair, breast enlargement

Testosterone injections can minimize feminized traits

Changes in Sex Chromosome Number XYY Symptoms- taller, may be mildly

retarded, basically phenotypically normal

Once thought to be genetically predisposed to be criminals