Lecture 2 Flashcards
Humans are \_\_\_\_\_ (2N) However, to progress through successive generations we need to create \_\_\_\_ (1N) gametes, that fuse to make diploid (2N) \_\_\_\_\_\_\_ at fertilization. Cell division is of two types: - \_\_\_\_\_ 1N to 2N - \_\_\_\_\_ 2N to 2N
Humans are diploid (2N) However, to progress through successive generations we need to create haploid (1N) gametes, that fuse to make diploid (2N) zygotes at fertilization. Cell division is of two types: - Meiosis 1N to 2N - Mitosis 2N to 2N
what is a chromosome
chromosomes are made of a single molecule of the DNA double helix packaged onto a protein scaffold
the genes are encoded in a linear fashion on the DNA
the protein scaffold hold the chromosomes together and has no coding function
describe differences between mitosis and meiosis
Meiosis: Two cell divisions Four haploid daughter cells Not identical to mother Produces gametes
Mitosis: One cell division Two daughter cells Daughters identical to mother All somatic cell divisions 
describe Genes on different chromosomes
Genes on different chromosomes are inherited independently.
If we cross two individuals that are heterozygous (+/-) at two genes that are on different chromosomes (the F1 cross), then the wild-type and mutant alleles at the two genes segregate independently (at random).
The outcome is a 9:3:3:1 ratio
describe Genes on the same chromosomes
Genes on the same chromosome tend to be co-inherited
The closer they are together on a chromosome the higher the frequency of co- inheritance
describe Patau’s Syndrome: Trisomy 13
90% of the patients do not survive beyond one year
Most children die before completing six months of age
describe Edward’s Syndrome: Trisomy 18
400 births per year in UK Most do not survive first year Features & characteristics: - delayed growth - feeding and breathing difficulties mental retardation - malformation of fingers & toes - heart & kidney defects
describe Downs Syndrome: Trisomy 21
Chromosomal Cause: - Extra Chromosome 21 (47ch.) - Often mosaic (mix of 46 & 47 ch.) Phenotype: - Short stature, mental retardation - Susceptible to Alzheimers Disease Risk factors: - Age of Mother <30 0.04%; >35 1.25% Pre-Natal Diagnostics: - Offered to women over 35 - 87/88 Glasgow pregnancies terminated
describe Turners syndrome
Single X chromosome
1 in 5000 births
Female phenotype
Often mosaic
describe Kleinfelters Syndrome: XXY
XXY (47) chromosomes
Affects one in 2000 births
Male phenotypes
If having an abnormal number of chromosomes is so deleterious then how do mammals cope with females having 2 X chromosomes and males having one X?
Because early in female development one X chromosome is switched off and forms an inactive Barr body
Consequence:
(1) Both sexes use one X chromosome
(2) Human females are genetic mosaics
what is disagree compensation
a mechanism to equalise the dosage of X chromosome gene products by means of inactivating one of the female X chromosomes
what are barr bodies like
Females have Barr Bodies
In females the inactivated X- chromosome is condensed and appears as a darkly staining body attached to the edge of the nuclear membrane. Barr Body Testing introduced for the 1968 Olympic Games.
describe x linked conditions in females vs males
females have 2 X chromosomes but only 1 is used. Linked mutations in heterozygous females generate mosaic tissues
males have 1 X-chromosome. X linked mutations always cause a mutant phenotype in a male
describe X inactivation
unfavourable skewing can cause female carriers to be affected.
X inactivation causes dosage of X-linked genes to be equalised between XX and XY
skewed X inactivation is defined as >80% of X chromosomes showing preferential inactivation of one chromosome
consistent relationship between pattern of X inactivation and clinical phenotype has been difficult to demonstrate
