Genetics Exam 2
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- 47, XXY
- genetalia and internal ducts male
- testes underdeveloped; no sperm.
- feminine sexual development is not entirely suppressed resulting in slight breast enlargement
- 2/1000 male births
- 45, X
- female external genetalia and internal ducts
- rudimentary ovaries
- short (<5 ft tall)
- webbed neck
- broad, shield-like chest
- 1/3000 female births
47, XXX Syndrome
- frequency 1/1200 female births
- highly variable in expression; some individuals are normal while others exhibit underdeveloped secondary sexual characters, sterility, and mental retardation.
Why does it matter if there are extra X chromosomes?
- disrupts the balance of genetic info
- too much gene product
- 47, XYY
- studied by Patricia Jacobs in 1935
- studied Scottish inmates in max security prison
- "criminal phenotype"
- in 1974, Walzer and Gerald wanted to follow XYY individual through lives but pressured to drop study
5th week of gestation
gonadal primordial tissue arises as a pair of ridges associated with each embryonic kidney
2. Mullerian ducts
- 1. undifferentiated male ducts
- 2. undifferentiated female ducts
7th week of gestation
- if the cells of the genital ridge have the XY constitution, development of the medullary region into testis is initiated
- if no Y-chromosome, the cortex forms ovarian tissue
12th week of gestation for females
the oogonia within the ovaries begin meiosis and primary ooccytes can be detected
25th week of gestation (females)
all oocytes become arrested in meiosis and remain dormant until puberty
Primary spermatocytes produced
when males reach puberty
- true ones are rare
- less extreme cases occur up to 2% of the time
- about 30 genetic and hormonal conditions can result in this
- Pseudoautosomal regions on Y-Chromosome
- shares homology with the x-chromosome
- Nonrecombining Region on y-chromosome
- 20 genes
- 12 genes fall into two groups: 5 have homologs and are expressed in a wide range of tissues, 7 lack x-homologs and are expressed only in testis.
- Sex determining region of the Y-chromosome
- this area encodes Testis Determining Factor
- Testis Determining Factor
- genes responsible for triggering undifferentiated gonadal tissue of the embryo to form testis
the actual proportion of male to female offspring
Primary Sex Ratio
reflects the proportion of males to females conceived in the population
Secondary Sex Ratio
reflects the proportion of males to females born in a popultion
In US, fetal mortality was higher in which sex according to a 1948 study?
What is the general sex ration in US Caucasians according to recent studies?
Barr and Bertram
worked with interphase nerve cells of cats and found dark staining body against nuclear envelope in females but not males
- inactivated X-chromosome
- provided a mechanism for dosage compensation of X-linked genes
Lyon and Russell
- studied X-chromosome deactivation
- found it randomly inactivated different X-chromosomes
- found that it occurs early in embryonic development
Mammalian females are mosaics for all heterozygous x-linked traits
Red-green color blindness
- support of Lyon hypothesis
- Hemizygous males fully color blind
- heterozygous females display mosaic retinas
Anhidrotic ectodermal dysplasia
- supports Lyon hypothesis
- hemizygous males show absences of teeth, sparse hair growth, and lack of sweat glands.
- Heterozygous females reveal patches of tissues without sweat glands.
supports Lyon hypothesis
Mechanism of X-inactivation
- XIST (X-Inactive Specific Transcript) gene withing the XIC (X-inactivation Center)
- expressed only on chromosomes to be inactivated.
Sex-determination in Drosophila
- Y-chromosome doesn't have male determining factors, but does contain info important to male fertility.
- some female determining factors are located on the X-chromosome.
Genetic Balance Theory
- a threshold for maleness is reached when X:A is 1:2
- additional X alters this balance and results in femaleness
Master switch for activation of a least 4 regulatory genes in Drosophila
activated when X:A and results in female development
- Mutations in Sxl mill female embryos but not males
Dosage Compensation in Drosophila
- no x-chromosome inactivation
- Male X-linked genes are transcribed at twice the level compared to X-linked genes in females.
- Dosage compensation is regulated by at least 4 autosomal male specific lethal genes, controlled by Sxl.
complete haploid sets of chromosomes are present
organism gains of losses one or more chromosomes but not a complete haploid set
3 or more complete haploid sets of chromosomes (Triploid - 3n, tetraploid = 4n, pentaploid = 5n, etc)
- loss of a single chromosome
- reveals lethal recessive alleles
- 46, -5p
- partial monosomy in humans
- only a part of a chromosome is lost: SEGMENTAL DELETION
- loss of 1/2 short arm chromosome 5
- anatomical malformations (cardiac, gastrointestinal, abnormal development of glottis and larynx)
- size of deletion influences severity
- trisomy 21
- 47, 21+
- usually due to nondisjunction during ovum formation
- shortened life expectansy (into their 50s)
- prone to respiratory disease, heart malformations, leukemia, and Alzheimer's disease.
- 47, 13+
- trisomy 13
- not mentally alert, cleft palage, polydactyly
- average life span 3 months
- trisomy 18
- 47, 18+
- small newborn
- elongated skulls
- webbed neck, congenital dislocation of hips, receding chim
- average survival time is 4 months with death usually caused by pneumonia or heart failure.
each additional set of chromosomes is identical to parent species.
- heat/cold shock during meiosis
- apply colchicine to somatic cells undergoing mitosis.
- 26 pairs of chromosomes
- allotetraploid AADD
- hybrid between American and Asian coton
cells in normal diploid organism becomes polyploid through ENDOMITOSIS
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