law of segregation

two alleles for a given character separate during gamete formation, and go to different gametes

law of independent assortment

alleles for different characters sort independently of each other during gamete formation

Gregor Mendel's "hereditary factors" and year introduced

(1860) Genes are segments of DNA located along chromosomes

Cytologists worked out the process of mitosis (year) and meiosis (year)

1875;1890s

chromosome theory of inheritance (year)(who)(definition)

• (1902) Walter S. Sutton, Theodor Boveri
• Genes occupy specific loci on chromosomes
• The chromosomes undergo segregation and independent assortment.

Thomas Hunt Morgan

Provided the first solid evidence of a specific gene associating with a specific chromosome.

• Drosophila Melanogaster
• (eye color)
• (body color, wing shape)

Morgans choice of experimental organism (spelled correctly) (# of chromosomes) (how often a new generation is bred)

Drosophila Melanogaster; 4; every 2 weeks

Wild type

the phenotype for most organisms

Morgans experimental conclusions

1. Fly's eye color linked to it's sex
2. Gene involved in white-eyed mutant was located on the X chromosome only

Which one of Mendel's laws relates to the inheritance of alleles for a single character?

law of segregation

Which one of Mendel's laws relates to the inheritance of alleles for two characters in a dihybrid cross?

law of independent assortment

What is the physical basis for each of Mendel's laws in meiosis?

the law of segregation: separation of homologs in anaphase I

the law of independent assortment: alternative arrangements of different homologous chromosome pairs in metaphase I

When does the anatomical signs of sex begin to emerge in humans?

when the embryo is about 2 months old

SRY

• Gene on the Y chromosome required for the development of testes.
• In the absence of SRY, the gonads develop into ovaries.
• SRY codes for a protein that regulates other genes.

Y-linked genes

78 genes code for about 25 proteins

X-linked genes (human)

1,100 genes

X-linked recessive disorders

1. color blindness
2. Duchenne muscular dystrophy
3. Hemophilia

hemizygous

term used in describing a males X-linked gene due to only one locus (XⁿY)

Duchenne muscular dystrophy

• 1/3500 males (US)
• progressive weakening of the muscles and loss of coordination
• absent muscle protein called dystrophin (gene for this protein located on X-chromosome)
• rarely live past early 20's

Hemophilia

• absence of one or more of the proteins required for blood clotting
• bleeding prolonged because clot is slow to form
• treated with intravenous injections of the protein that is missing

XIST

• Inactivation of an X chromosome involves modification of the DNA and proteins bound to it, called histones, including attachment of methyl groups (--CH₃) to DNA nucleotides
• Two regions, one on each X chromosome, associate briefly with each other in each cell
• Gene called XIST (X-inactive specific transcript) becomes active only on the chromosome that will become the Barr body

Barr body

A compact object of condensed inactive X found on the inside of the nuclear envelope.

Mosaicism

When an organism has cells with more than one genotype

Tortoiseshell cat

• Tortoiseshell gene is on the X chromosome
• Tortoiseshell phenotype requires the presence of two different alleles, one for orange fur and one for black fur.
• Only females can have both alleles, because only they have two X chromosomes.
• Orange patches are formed by populations of cells in which the X chromosome with the orange allele is active; black patches have cells in which the X chromosome with black allele is active.

linked genes

genes located near each other on the same chromosome tend to be inherited together

genetic recombination

the production of offspring with combinations of traits that differ from those found in either P generation parent.

parental types

an offspring whose phenotype matches one of the true-breeding parental phenotypes

recombinant types

an offspring whose phenotype differs from that of the true-breeding parental phenotypes

50% or greater frequency of occurrence of parental types indicates two genes are on the same chromosome.

crossing over

end portions of two nonsister chromatids trade places; recombination of linked genes

genetic map (who)(what)

Alfred H. Sturtevant; an ordered list of the genetic loci along a particular chromosome

recombination frequency

the percentage of recombinant offspring; depends on the distance between genes on a chromosome

the farther apart two genes are, the higher the probability that a crossover will occur between them and therefor the higher the recombination frequency

linkage map

a genetic map based on recombination frequencies

cinnabar

one of many Drosophila genes affecting eye color

map units

the distances between genes; one map unit is equivalent to a 1% recombination frequency

cytogenetic maps

locate genes with respect to chromosomal features, such as stained bands, that can be seen in the microscope

nondisjunction

the members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separate during meiosis II

aneuploidy

an aberrant gamete that unites with a normal gamete at fertilization

monosomic

2n-1; missing chromosome in zygote resulting from fertilization of normal gamete with one that has no copy of a particular chromosome

trisomic

2n+1; extra chromosome in zygote resulting from fertilization of normal gamete with one that has an extra copy of a particular chromosome

Monosomy and trisomy are estimated to occur in between 10 and 25% of human conceptions, and is the main reason for pregnancy loss

polyploidy

more than two complete chromosome sets in all somatic cells

triploidy

3n; three chromosome sets; may arise by the fertilization of an abnormal diploid egg produced by nondisjunction of all its chromosomes

tetraploidy

4n; four chromosome sets; may arise from the failure of a 2n zygote to divide after replicating its chromosomes

examples of polyploid

bananas (triploid, 3n); wheat (hexaploid, 6n); strawberries (octoploid, 8n)

polyploids appear more normal than aneuploids. One extra (or missing) chromosome apparently disrupts genetic balance more than does an entire extra set of chromosomes.

2 ways that lead to changes in chromosome structure

1. errors in meiosis
2. radiation can cause breakage of chromosomes

4 types of alterations of chromosome structure

1. deletion
2. duplication
3. inversion
4. translocation

deletion

chromosomal fragment is lost

duplication

the "deleted" fragment may become attached as an extra segment to a sister chromatid

inversion

a chromosomal fragment may also reattach to the original chromosome but in the reverse orientation

reciprocal translocation

most common; nonhomologous chromosomes exchange fragments

nonreciprocal translocation

less common; a chromosome transfers a fragment but receives none in return

translocations and inversions can alter phenotype because a gene's expression can be influenced by its location among neighboring genes

syndrome

a set of traits characteristic of the type of aneuploidy

down syndrome

• trisomy 21 (extra chromosome 21)
• 1/830
• characteristic facial features, short stature, correctable heart defects, developmental delays
• increased chance of leukemia & Alzheimers
• lower rate of high blood pressure, atherosclerosis, stroke & many types of solid tumors
• life span: shorter than normal
• all males and half females are sexually underdeveloped & sterile
• frequency increases with mothers age (0.04% under 30)(0.92% at 40)
• prenatal screening available

Prenatally and Postnatally Diagnosed Conditions Awareness Act (when)(what)

2008; law stipulates that medical practitioners give accurate, up-to-date information about any prenatal or postnatal diagnosis received by parents and that they connect parents with appropriate support services

aneuploidy of sex chromosomes

• XXY (Klinefelter syndrome)
• XYY
• Trisomy X (XXX)
• Monosomy X (Turners syndrome)

XXY

• Klinefelter syndrome
• 1/500-1000 male births
• testes are small; sterile
• breast enlargement and other female body characteristics
• subnormal intelligence

XYY

• 1/1000 males
• normal sexual development
• no well defined syndrome
• taller than average

XXX

• trisomy X
• 1/1000 female births
• healthy
• taller than average
• at risk for learning disabilities
• fertile

XO

• monosomy X (Turners syndrome)
• 1/2500 female births
• only known viable monosomy in humans
• sterile
• estrogen replacement therapy will develop secondary sex characteristics
• normal intelligence

disorders caused by structurally altered chromosomes

• cri du chat ("cry of the cat")
• CML

cri du chat

• deletion in chromosome 5
• severely intellectually disabled
• small head with unusual facial features
• cry that sounds like mewing of distressed cat "cry of the cat"
• lifespan: infancy or early childhood

CML

• chronic myelogenous leukemia
• reciprocal translocation during mitosis of cells that will become white blood cells
• exchange of a large portion of chromosome 22 with a small fragment of time of chromosome 9
• produces a Philadelphia chromosome (shortened chromosome 22)
• causes caner by activating a gene that leads to uncontrolled cell cycle progression

genomic imprinting

variation in phenotype depending on whether an allele is inherited from the male or female parent (most imprinted genes are on autosomes)(over 60 imprinted genes have been identified, with hundreds more suspected)

What exactly is a genomic imprint?

• in many cases, it seems to consist of methyl (-CH₃) groups that are added to cytosine nucleotides of one of the alleles (methylation)
• methylation of certain cytosines on the paternal chromosome leads to expression of the paternal Igf2 allel, by an indirect mechanism involving chromatin condensation

not all of a eukaryotic cell's genes are located on nuclear chromosomes, or even in the nucleus; some genes are located in organelles in the cytoplasm

extracellular genes or cytoplasmic genes

genes outside the nucleus

organelle genes are not distributed to offspring according to the same rules that direct the distribution of nuclear chromosomes during meiosis, so they do not display mendelian inheritance (p. 309)

Carl Correns (year)

1909; Discovered that extranuclear genes exist; studied the inheritance of yellow or white patches on leaves of a green plant

parts of body most susceptible to energy deprivation

nervous system, muscles

mitochondrial diseases

1. mitochondrial myopathy
2. Leber's hereditary optic neuropathy

mitochondrial myopathy

• mitochondrial disorder
• causes weakness, intolerance of exercise, and muscle deterioration

Leber's hereditary optic neuropathy

• mitochondrial disorder
• can produce sudden blindness in people as young as their 20's or 30's
• the four mutations found thus far to cause this disorder affect oxidative phosphorylation during cellular respiration

mitochondrial mutations inherited from a person's mother

• some types of diabetes, heart disease, and Alzheimer's
• believed to be a part of the normal aging process