3- Genetics Flashcards
Who is coined as the «father of genetics?»
Gregor Mendel
Why were peas used?
-other research showed that pea hybrids could be produced
-many pea varieties were available
-peas are small plants, fast and easy to grow
-peas can self fertilize or be cross-fertilized
What are the 3 steps of Mendel’s experimental method?
1) Produce true-breeding strains for each trait he was studying
2) Cross-Fertilize true-breeding strains having alternate forms of a trait
3) Allow the hybrid offspring to self-fertilize for several generations and count the number of offspring showing each form of the trait
What are true-breeding strains?
Variety which over multiple generations of self crossing give descendants with characteristics identical to the parent plant
What are the 7 different traits Mendel produced for true-breeding pea strains?
1)Flower colour
2) Seed colour
3) Seed texture
4) Pod colour
5) Pod shape
6) Flower position
7)Plant height
How many variants does each trait have?
2
How does pollen get to the female gametes (ovary)?
The pollen is carried from the anthers to the stigma (a sticky organ that traps pollen and allows pollen to move down the pistil) to the female gametes (ovary)
What did Mendel do to prevent the pea plant from self-fertilizing and confounding his results?
He removed all of the anthers from the plant’s flowers before they had the chance to mature.
What is ‘’reciprocal cross’’?
Male and female cross give the same type of offspring then female and male cross
What is the F1 generation?
First filial generation is the offspring produced by crossing 2 true-breed strains
Does blending of traits occur?
No
What is the F2 generation?
Second filial generation are the offspring resulting from the self-fertilization of F1 plants
What is the phenotypic ratio for F2 generation?
3:1 of dominant:recessive
What is the phenotypic ratio of of F3 generation?
1:2:1
-1 true-breeding dominant plant
-2 not true-breeding dominant plants
-1 true-breeding recessive plant
What is Mendel’s five-element model?
1) Parents transmit discrete factors
2) Each individual receives one copy of a ‘’gene’’ from each parent
3) Not all copies of a gene are identical
4) Alleles remain discrete, no blending
5) Presence of allele does not guarantee expression
(Dominant/recessive allele)
Define the following:
-Allele
-Homozygous
-Heterozygous
Allele: alternative form of a gene (flower purple or white)
Homozygous: 2 of the same allele
Heterozygous: different alleles
What is phenotype and genotype?
Phenotype: physical appearance
Genotype: total set of alleles an individual contains
What is Mendel’s Law of Segregation?
-The law states that paired unit factors (gene alleles) must segregate equally into gametes such that offspring have an equal likelihood of inheriting either factor
-Mendel had no knowledge of chromosomes, genes, alleles or meiosis, since it had not yet been described at the time
What was Mendel’s Law of Independent Assortment?
-Genes do not influence each other with regard to the sorting of alleles into gametes, and every possible combination of alleles for every gene is equally likely to occur when they are located on different chromosomes.
How many types of gametes does each true-breeding parent make?
Only one type of gamete
What is ‘’test cross’’?
An individual with unknown genotype is crossed with the homozygous recessive genotype
What are dihybrid crosses?
-Examination of 2 separate traits in a single cross (genes located on different chromosomes)
-Produced true-breeding lines for 2 traits
The F1 generation of dihybrid cross shows only the ________ phenotypes for each trait
Dominant
(In Dihybrid crosses) The F2 generation shows all four possible phenotypes in a set ratio of _____
9:3:3:1
What are the principles of independent assortment?
- In dihybrid cross, the alleles of each gene assort independently
- The segregation of different allele pairs is independent
-Independent alignment of different homologous chromosome pairs during metaphase I leads to the independent segregation of the different allele pairs
-I.E: in other words the genes that we are testing are on different chromosomes
What did Walter Sutton do?
-Chromosomal theory of inheritance
(Based on observations that similar chromosomes paired with one another during meiosis (homologous chromosomes)
Who is Thomas Hunt Morgan?
-Was an American evolutionary biologist, geneticist, embryologist and science author
-Won the Nobel Prize in Physiology or Medicine in 1933 for discoveries elucidating the role that the chromosome plays in heredity
-Working with fruit fly
What did Thomas Hunt Morgan discover?
A mutant male fly with white eyes instead of red
How did Thomas Hunt Morgan test Mendelian inheritance?
He crossed the mutant male to a normal red-eyed female
Typically females have __ homologous chromosomes whereas a male will have __
1) 23
2) 22
Why do females have more homologous chromosomes than males?
The sex chromosomes X and Y are not homologous but instead hemizygous
In humans and Drosphile (flies..) the male is __________ for the sex chromosomes (the chromosomes are different)
Hemizygous
In birds, the female is ______ for the sex chromosomes
Hemizygous (the chromosomes are different (ZW))
How is the sex chromosomes for grasshopper males?
They lack the second sex chromosomes (XO)
In honeybees, the female are _______ (2n)
While the male are ____ (n)
1) Diploid
2) Haploid
True or false?:
The ‘’default’’ setting in human embryonic development leads to male development
False, it leads to female development
Some genes on which chromosome are responsible for the masculinization of genitalia and secondary sex organs?
Y
True or false?:
There is an inactivation of one of the X chromosomes in females
True.
Which of the two X inactivated is done randomly throughout the body
What is a Barr body?
Inactivated X chromosome that is highly condensed, making it visible using staining and is located in nucleus attached to the nuclear membrane
Does Barr body lead to total inactivation of one of the X chromosomes in females?
NO (partial/most)
The colour of calico cats’ fur is coded on which chromosome?
The X chromosome
True or False: female population would be striped/spotted if we can see the inactivation of one of the X chromosomes
True
Define inheritance
Inheritance is the passage of hereditary traits from one generation to the next
How many chromosomes do you get per parent?
23 from mother and 23 from father
In which phase of meiosis can homologous chromosomes physical ally exchange material by crossing over?
In prophase I
-This is done between homologous chromosomes (from both parents), not sister chromatids (DNA copy).
-There can be multiple cross overs on one chromosome
What is a recombinant?
A gamete with the crossover chromosome
The frequency for 2 particular alleles to cross over and change from one homologous chromosome to another is dependant on what?
Its physical distance on the chromosome
Are all genes on the sex chromosomes related to sex characteristics?
No
Is red-green colour blindness more common in males or females?
Males because it is carried on the X chromosome
What is hemophilia?
A disease that affects a single protein in a cascade of proteins involved in the formation of blood clots
What is albinism?
A disorder where recessive alleles are mutated and its characterized in humans by the complete or partial absence of pigment in the skin, hair and eyes.
-Long thought to be due to a single gene, albinism is now known to result from mutations in multiple genes; the common feature is the loss of pigment from eyes and hair.
-The genes are present on the autosome chromosome, thus females and males are affected equally
The failure of homologous or sister chromatids to separate properly during meiosis is called _______?
Nondisjunction
What is aneuploidy?
Individuals who have an abnormal number of chromosomes
What is monosomic?
Individuals who have lost one copy of an autosome. Generally do not survive embryonic development
What is trisomies?
Individuals who have gained an extra autosome
Extra autosome from chromosomes 13,15,18, are viable at birth, but die within a few months.
Extra autosome for chromosomes 21 (Down syndrome) survive to adulthood. In these individuals, the maturation of the skeletal system is delayed, and have poor muscle tone. Their mental development is also affected
What is amniocentesis?
Removing a small sample of amniotic fluid which contains fetal cells that can be grown in culture for analysis (is a genetic test performed when a pregnancy is considered high risk)
What is chorionic villi sampling?
A less invasive method of amniocentesis, which involves removing cells from the chorion, a membranous part of the placenta that nourishes the fetus
Problems in meiosis can lead to gametes with _______ or an extra amount.
No sex chromosomes
What is XXX, XXY, XO, OY and XXY?
XXX = triple X females
XXY males = Klinefelter syndrome
XO females = Turner syndrome
OY = not viable (will not survive)
XYY males = Jacob syndrome
What are the characteristics of Triple X females (XXX)?
-Generally appear normal both physically and mentally
-Widely spaced eyes
-Are fertile
-Slight increased risk of mental retardation
-Even women with 4+ X chromosomes are generally normal
What are characteristics of Klinefelter syndrome?
-Development of small testicles
-Infertility
-Lower levels of male sex hormones
-Female-type pubic hair pattern
What are characteristics of Turner syndrome (Female XO)?
-Shorter than normal
-Broad chest, low hairline and webbed necks
-Dysfunction of the ovaries (sterile)
-Puberty affected (may not undergo puberty)
- Most suffer from heart disease
What are characteristics of Jacob syndrome (male XYY)?
-Not many unusual physical differences (some slightly taller and thinner)
-Normal fertility
-Some have more severe acne than normal
-Increased risk of learning and reading difficulties
What is a pedigree?
-A pedigree is a family tree that describes the interrelationships of parent and children across generations
-Inheritance patterns of particular traits can be traced and described using pedigrees
How do you read a pedigree?
-Male are squares and females are circles
-Normal individuals are white and affected individuals are coloured or black
-Horizontal line linking the centre of a square and circle represent mating
-Offspring of this mating are linked to their parents by horizontal and vertical lines
What are three examples (types) of pedigree?
1) Dominant trait pedigree
2) Recessive trait pedigree
3) Sex-link trait pedigree
What is an example of dominant trait pedigree?
Juvenile glaucoma (autosome chromosome)
-Disease that causes degeneration of optic nerve, leading to blindness
-Males and females are affected equally, since it’s on an autosome chromosome
What is an example of recessive trait pedigree?
Albinism
-Caused by the nonfunctional allele of the enzyme tyrosinase
-Males and females are affected equally, since it’s on an autosome chromosome
-Most affected individuals have unaffected parents
-The carrier (half colour) are not always present in a pedigree
What is an example of sex-link trait pedigree?
Hemophilia (sex chromosome)
-It’s an inherited genetic disorder that impairs the body’s ability to make blood clots
-The mutations causing the disease is a recessive allele located on the X-sexual chromosome (sex-linked)
-Males are more affected than females
What does Mendel’s model of inheritance assume?
-Each trait is controlled by a single gene
-Each gene has only 2 alleles (ex.: purple or white)
-There is a clear dominant-recessive relationship between the alleles
Do most genes follow Mendel’s model of inheritance?
No!
The ranges of dominance and recessive relationships
1) Complete dominance
2) Incomplete dominance
3) Codominance
What is codominance and give an example
-Most genes in a population possess several different alleles, and often no single is dominant; instead, each allele has its own effect, and the heterozygote shows some aspect of both allele in their phenotype. The alleles are said to be codominant
-Ex: Human ABO blood group (3 alleles of the I gene)
What are the 4 extensions to the Mendel model (ranges ordinance and recessive relationships)?
1) Polygenic inheritance
2) Pleiotropy
3) Epistasis
4) Environmental
What is polygenic inheritance?
-Occurs when multiple genes are involved in controlling the phenotype of a trait.
-The phenotype is an accumulation of contributions by multiple genes
-These traits show continuous variation and are referred to as quantitative traits
Ex: human height
What is the main difference between multiple alleles and polygenic inheritance?
Multiple alleles: more than 2 alternative forms of a single gene that determines a trait
Polygenic inheritance: polygenic traits are determined by several genes
What is pleiotropy?
-Refers to an allele which has more than one effect on the phenotype
-Pleiotropic effects are difficult to predict, because a gene that affects one trait often performs other, unknown functions
Ex: gene responsible for the blue colour of a cat’s eye also makes it deaf in one ear
How does pleiotropy affect flies?
Drosophila (the vestigial gene) plays a critical role in wing development
-Homozygous flies for the recessive form of the vestigial gene (vg), will develop short wings
- The vg gene is also pleiotropic:
1) Changes the number of egg strings in a fly’s ovaries
2) Alters the position of bristles on a fly’s scutellum
3) Decreases the length of a fly’s life
How does pleiotropy affect chickens?
Chickens that have the frizzle trait have:
-Abnormal body temperatures
-Higher metabolic and blood flow rates
-Laid fewer eggs
What is epistasis?
The expression of one gene can affect the phenotypic expression of another (ex: the colour of a Labrador dog depends on pigment but also on the gene for the pigment deposition)
What is an example of epistasis?
-Crossed two true-breeding corn varieties, each lacking the anthocyanin pigment.
-Epistasis was involved, since the colour emerged from the action of two enzymes (a AND b). If one is absent, the seed is colourless
What is environmental influence (extension to Mendel model)
Phenotypes may be affected by the environment
Ex: coat colour in Himalayan rabbits and Siamese cats
-Allele produces an enzyme that allows pigment production only at temperatures bellow 33 degrees
-Hydrangea flower colour change according to soil pH
