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Inheritance
Mendelian Genetics
Traits are transmitted from parents to offspring
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He discovered the principles of heredity by breeding garden peas in
experiments
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He could control which plants mated with
which
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Before, they believed in the ‘blending hypothesis’ which is that every offspring has a
blend of traits from its parents
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Before experiments, he removed the stamens from the flowers to avoid selfpollination when it was not required
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When true-breeding
plants self-pollinate, all their offspring have the same traits
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Mendel’s experiments
revealed two fundamental principles of heredity: the law of segregation and the law
of independent assortment
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He also discovered that one trait was usually dominant whilst another was
usually recessive
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1
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E
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the gene for flower colour in pea-plants exists in two versions; purple
and white
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2
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A diploid organism
inherits one set from each parent
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3
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The recessive has no effect on the appearance
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Mendel’s law of segregation states that the two alleles for a heritable
character separate and segregate during gamete production and end up in
different gametes
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A Punnett square predicts the results of a genetic cross between individuals of
known genotype
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When the alleles are
different, the organism is heterozygous
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All F1 progeny produced in these crosses were monohybrids, heterozygous for one
character
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In this type of cross he observed the phenotypic ratio of 9:3:3:1
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Mendel’s laws of independent assortment and segregation explain heritable
variation in terms of alternative forms of genes that are passed along according to
simple rules of probability
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•

Incomplete dominance – heterozygotes show distinct intermediate
phenotype not seen in homozygotes (when the heterozygous condition
expresses a third phenotype, different from either homozygous conditions
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g
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Multiple Alleles
The ABO blood groups in humans are determined by three alleles, IA, IB and i
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IA and IB are codominant to each other
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Pleiotropy
This is when one gene has multiple effects on the physiology of an organism
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Epistasis
This is when a gene at one locus alters the phenotypic expression of a gene at a
second locus
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Polygenic inheritance
It is the additive effects of two or more genes on a single phenotypic character
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Individuals with intermediate skin shades
will be most common, but some very light and very dark individuals could be

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produces as well
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A person becomes darker if they tan, despite their inherited skin colour
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Inheritance of Recessive Human Genetic Disorders
These conditions range from relatively mild (albinism) to life-threatening (cystic
fibrosis)
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If these channels are defective or absent, there
are abnormally high extracellular levels of chloride
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This mucus builds up in the
pancreas, lungs, digestive tract, and causes poor absorption of nutrients, chronic
bronchitis and bacterial infections
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When
oxygen levels in the blood of an affected individual are low, sickle-cell haemoglobin
aggregate into long rods that deform RBCs into a sickle shape
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Individuals with sickle-cell allele have increased resistance to malaria
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Inheritance of Dominant Human Genetic Disorders
An example of this is achondroplasia (form of dwarfism)
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If a lethal dominant kills an offspring
before it can mature and reproduce, the allele will not be passed on to future
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generations
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A lethal dominant allele can escape elimination if it causes death at a relatively
advanced age, after the individual has already passed on the lethal allele to his or
her children
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The dominant lethal allele has no obvious phenotypic effect
until an individual is about 35 to 45 years old
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Multifactorial Diseases
These may have a genetic component plus a significant environmental influence
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Chromosomal Genetics
Chromosome theory of inheritance:
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Genes occupy specific loci on chromosomes

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Chromosomes undergo segregation during meiosis

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Chromosomes undergo independent assortment during meisois

Morgan’s Experiments
He worked on Drosophila melanogaster, a fruit fly that eats fungi on fruit
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They have three pairs of
autosomes and a pair of sex chromosomes
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All the F1 generation had red eyes, suggesting that the red allele was dominant to
the white allele
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The white-eyed trait appeared only in F2 males,
suggesting that the fly’s eye colour was linked to its sex
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Males have only a single allele
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Linkage
Genes located on the same chromosome that tend to be inherited together are
called linked genes
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B+ and vg+ are
dominant
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The other two phenotypes were fewer than expected from independent assortment
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Parental types: phenotypes that match the original P parents
Recombinants: new combinations of parental traits
Linkage mapping

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The percentage of recombinant offspring, the recombination frequency, is related to
the distance between linked genes
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Map units indicate
relative distance and order, not precise locations of genes
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An individual who inherits
two X chromosomes usually develops into a female
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Other animals have different methods of sex determination
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Females are XX and males are X
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In bees and ants, females are
diploid and males are haploid
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Only
short segments on the Y chromosome are homologous with the corresponding
regions of the X chromosomes
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There is a gene on the Y chromosome which is required for the
development of testes
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The generic embryonic gonads develop into testes
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Other genes on the Y chromosome are necessary for the production of functional
sperm
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In individuals lacking the SRY gene, the generic embryonic gonads develop into
ovaries
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a
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When the mother is dominant
homozygote, the daughters will have the normal phenotype
but will be carriers of the mutation
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If a carrier mates with a male of normal phenotype, there is
50% chance that each daughter will be a carrier and a 50%
chance that each son will have the disorder
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If a carrier mates with a male who has the disorder, there is
a 50% chance that each child born will have the disorder
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Males are far more likely to exhibit sex-linked recessive disorders tan are females
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These individuals have prolonged bleeding because a
firm clot forms slowly
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Today they are treated with intravenous injections of the missing
protein)
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Most of the genes on the Barr body are not expressed
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For example the orange-andblack pattern on tortoiseshell cats is due to patches of cells
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Errors
during meiosis can alter chromosome number in a cell
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This occurs when tetrad chromosomes do not separate properly
during meiosis I and when sister chromatids fail to separate during meiosis II
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Offspring resulting from fertilization of a normal gamete with one produces by
nondisjunction will have an abnormal chromosome number; aneuploidy
Trisomic cells have 3 copies of a particular chromosome type and have 2n+1 total
of chromosomes
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Polyploidy is when the organism has more than two complete sets of
chromosomes, this occurs when a normal gamete fertilizes another gamete in which
there has been nondisjunction of its entire chromosomes
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Alterations of chromosome structure
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Deletion: a chromosome fragment lacking a centromere is lost during cell
division, it would be missing certain genes

-

Duplication: a fragment becomes attached to an extra segment to a sister
chromatid
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-

Inversion: a chromosomal fragment reattaches to the original chromosome,
but in the reverse orientation
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Deletions and duplications are especially likely to occur during meiosis
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Duplications and translocations are typically harmful
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This can be diagnosed before birth by fetal testing
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They have
characteristic facial features, short stature, heart defects, susceptibility to respiratory
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infection, mental retardation, and increased risk developing leukemia and
Alzheimer’s disease
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These result from nondisjunction during gamete production in one parent
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This may be
linked to some age-dependent abnormality in the spindle checkpoint during meiosis
I, leading to nondisjunction
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This aneuploidy upsets the genetic balance less severely that autosomal
aneuploidy, because the Y chromosomes contains relatively few genes and because
extra copies of the X chromosomes become inactivated as Barr bodies in somatic
cells
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These have normal intellingence
♥ XYY; taller than average
♥ XXX; healthy females
♥ XO - Turner syndrome; phenotypically female but sterile because their sex
organs do not mature
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Structural alterations of chromosomes can also cause human disorders
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They are mentally
retarded , have small heads with unusual facial features and have a cry like the
mewing of a distressed cat
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The Chi Square Statistic
It allows us to compare a collection of categorical data with some theoretical
expected distribution
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33
4
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33

4
9
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07

Therefore degrees of freedom = n-1 = 2-1 =1
Chi Square distribution level:
Df
0
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841

2
5
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815

The Chi Square value is higher than the critical value therefore the null hypothesis is
rejected

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