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Diploid
A cell that contains pairs of homologous chromosomes, represented by 2n
Haploid
A cell that contains half the number of chromosomes as the parent cell, represented by n
Selective Breeding
The process by which humans use animal and plant breeding to develop particular phenotypic traits by choosing which males and females will sexually reproduce
Crossing Over
The exchange of chromosomal segments between a pair of homologous chromosomes at a chiasma during prophase I
Nondisjunction
The failure of homologous chromosome pairs or sister chromatids to separate during the anaphase stages in meiosis
Mitosis
The stage of the cell cycle where a cell’s nucleus and genetic material divide
Phases of Mitosis
Interphase occurs before mitosis to replicate the chromosomes.
Prophase
Metaphase
Anaphase
Telophase
Interphase
The stage of the cell cycle during which a cell carries out its normal functions, grows, and makes copies of its genetic material in preparation for the next stage of the cycle. The cell grows, matures, copies its DNA, and prepares for division
Growth 1 is the major period of growth where the cell synthesizes many new molecules
Synthesis is the period where DNA is replicated as uncondensed fibres called chromatin
Growth 2 involves the production of more molecules
Prophase
The stage where the cell’s chromatin condenses into chromosomes, the nucleus and nucleolus disappear, and spindle fibres form from the centrosomes and move towards the poles
Chromosome
A structure in the nucleus that contains DNA
Sister Chromatid
One of two chromosomes that are genetically identical and held together at the centromere
Centromere
The region where two sister chromatids are held together in a chromosome
Spindle Fibre
A microtubule structure that facilitates the movement of chromosomes within a cell
Centrosome
A structure that helps to form the spindle fibres
Metaphase
The stage where the spindle fibres from each pole attach to the centromere and guide the chromosome to the equator of the cell
Anaphase
The stage where each centromere splits apart, and sister chromatids, now individual chromosomes, are separated to opposite poles so that each pole has a complete set of DNA
Telophase
The stage that begins when chromosomes reach the opposite ends of the cell, where chromosomes start to unwind, spindle fibres break down, and the nucleus and nucleolus reform
Cytokinesis
The stage of the cell cycle that involves the division of the cell cytoplasm and creation of two new daughter cells
Meiosis
The cellular process that produces cells containing half the number of chromosomes as the parent cell (genetic reduction) and are genetically unique through combinations of alleles (genetic recombination)
Interphase Before Meiosis
Cells that will divide by meiosis will proceed through growth and synthesis, and replicate their chromosomes, so that the meiosis begins with duplicated chromosomes
Phases of Meiosis
Interphase happens before meiosis
Prophase I
Each pair of homologous chromosomes condense and line up side by side in synapsis, where genetic information is exchanged through crossing over. This is the longest meiotic phase
Tetrad
Two chromosomes, or four chromatids
Chiasma
The site where chromosomes cross over
Metaphase I
The pairs of homologous chromosomes (tetrads) line up along the equator of the cell, and spindle fibres attach to the centromere of each homologous chromosome. This is the shortest meiotic phase
Anaphase I
The homologous chromosomes separate and move to opposite poles, which turns a diploid cell into a haploid cell. Sister chromatid remain attached
Telophase I
Homologous chromosomes begin to uncoil and the spindle fibres disappear, the nuclear membrane forms, and two haploid cells form through cytokinesis
Prophase II
The nuclear membrane disappears and spindle fibres reappear
Metaphase II
A haploid number of chromosomes line up at the equator
Anaphase II
Sister chromatids are pulled apart at the centromere by the spindle fibres towards opposite poles of the cells
Telophase II
The nuclear membrane and nuclei reform
Cytokinesis After Meiosis
The two cells that have undergone telophase II split once more to form four unique haploid cells
Significance of mitosis
It allows cells to grow, repair, and replace other cells.
Significance of meiosis
It forms genetically distinct haploids through genetic recombination (independent assortment, random selection, and crossing over). The genetic uniqueness allows populations to have a greater variety in genes. The haploid cells allow two breeding individuals to form a diploid zygote, allowing for greater genetic diversity through sexual reproduction.
Independent Assortment
The orientation of each chromosome in a homologous pair to one pole, which can result in a variation of possible gametes containing a combination of maternal and paternal chromosomes
Monohybrid Cross
A cross of two individuals that differ by one trait
Mendelian Ratio
An approximate ratio of 3:1 dominant to recessive phenotypes in a monohybrid cross between two heterozygous parents
Dihybrid Cross
A cross of two individuals that differ in two traits due to two different genes
Steps of solving a dihybrid cross
Write a legend of each present allele
Write and box the parent line (the cross of each genotype)
Illustrate, with arrows, the possible gametes produced by each parent
Create a Punnett Square with the gametes of one parent along the top, and the gametes of the other along the side. The cross of each goes in the boxes.
Express the genotypes and phenotypes through ratios, fractions, or percents
Genotype ratios of a dihybrid cross between two heterozygotes
1 AABB
2 AABb
2 AaBB
4 AaBb
1 AAbb
2 Aabb
1 aaBB
2 aaBb
1 aabb
Phenotype ratios of a dihybrid cross between two heterozygotes
9 A_B_ (dominant, dominant)
3 A_bb (dominant, recessive)
3 aaB_ (recessive, dominant)
1 aabb (recessive, recessive)
Polygenic Traits
Traits controlled by multiple alleles
Codominant Genes
Two different alleles at a locus are responsible for different phenotypes, and both alleles affect the phenotype of the heterozygote
Solving dihybrid crosses with codominant genes
Solve as normal. The heterozygote of codominant alleles will express both as dominant (ex. Type AB blood is made of codominant IA and IB alleles)
Incomplete Dominance
The phenotype of a heterozygote is an intermediate of both the dominant and recessive traits
Solving dihybrid crosses with incomplete dominance
Solve as normal. The heterozygote will be a new phenotype (ex. in flowers, crossing a red and a white pigment allele will create a pink offspring)
Sex-Linked Genes
A gene coded on a sex chromosome
Solving dihybrid crosses with a dominant X-linked gene
In a monohybrid cross, the gametes come from XX and XY. Only the cross of an affected X chromosome will carry and express the gene. The phenotypes are:
XX - Normal female
XAX or XAXA - Affected female
XY - Normal male
XAY - Affected male
Solving dihybrid crosses with a recessive X-linked gene
In a monohybrid cross, the gametes come from XX and XY. Only the cross of an affected X chromosome will carry the linked gene, but all X chromosomes must be affected to express it. The phenotypes are:
XX - Normal female
XaX - Carrier female
XaXa - Affected female
XY - Normal male
XaY - Affected male
Lethal Alleles
A mutated gene that is capable of causing death
Solving dihybrid crosses with a dominant lethal allele
Solve normally. Both heterozygotes and dominant homozygotes will die. In a cross of heterozygotes, the ratio is 1 living: 3 affected/dead.
Solving dihybrid crosses with a recessive lethal allele
Solve normally. The homozygous lethal allele genotype will die, and the heterozygote (1 lethal, 1 normal) will express an abnormal phenotype. A cross between two heterozygotes will have a ratio of 1 dead: 2 abnormal: 1 normal
Complementary Genes
Genes can only be expressed in the presence of other genes
Solving dihybrid crosses with complementary genes
Solve normally. If a trait requires two different alleles to be dominant to be expressed, A_bb would be the recessive phenotype, aaB_ would be the recessive phenotype, and A_B_ would be the dominant phenotype. The ratio for a cross of two completely heterozygous parents is 9:7 for dominant to recessive.
Epistasis
One gene masks the expression of a different gene for a different trait
Solving dihybrid crosses with epistasis
Solve normally. If the genotype bb masks a gene, A_bb would not show the A trait, aaB_ would show the recessive phenotype for gene A, and A_B_ would show the dominant phenotype for A
Antigen/Agglutinogen
The proteins on the surface of a red blood cell that indicates the blood type.
Antibodies/Agglutinin
Proteins in the blood that can attack specific antigens, causing agglutination and hemolysis, which limits compatibility
Blood type compatibility
Blood types cannot receive blood with antigens that it has antibodies against
Genotype of type A blood
IAIA or IAi
Genotype of type B blood
IBIB or IBi
Genotype of type AB blood
IAIB
Genotype of type O blood
ii
Rh Factor
A type of antigen on blood cells, indicated by a + or -
Genotype for Rh+ blood
RR or Rr
Genotype for Rh- blood
rr
Hemolytic disease
An Rh- mother carries an Rh+ baby, and becomes sensitizes to the Rh+ (she produces antibodies). If the mother carries a second child with Rh+, the antibodies will attack the baby
Type A+
Agglutinogens: A, Rh
Donates to: A+, AB+
Agglutinins: B
Receives from: A+, A-, O+, O-
Type A-
Agglutinogens: A
Donates to: A+, A-, AB+, AB-
Agglutinins: B, Rh
Receives from: A-, O-
Type B+
Agglutinogens: B, Rh
Donates to: B+, AB+
Agglutinins: A
Receives from: B+, B-, O+, O-
Type B-
Agglutinogens: B
Donates to: B+, B-, AB+, AB-
Agglutinins: A, Rh
Receives from: B-, O-
Type AB+
Agglutinogens: A, B, Rh
Donates to: AB+
Agglutinins: NONE
Receives from: A+, A-, B+, B-, AB+, AB-, O+, O- (UNIVERSAL RECEIVER)
Type AB-
Agglutinogens: A, B
Donates to: AB+, AB-
Agglutinins: Rh
Receives from: A-, B-, AB-, O-
Type O+
Agglutinogens: Rh
Donates to: A+, B+, AB+, O+
Agglutinins: A, B
Receives from: O+, O-
Type O-
Agglutinogens: NONE
Donates to: A+, A-, B+, B-, AB+, AB-, O+, O- (UNIVERSAL DONOR)
Agglutinins: A, B, Rh
Receives from: O-
Karyotype
A photograph of pairs of homologous chromosomes in a cell
Karyotype Notation
# of chromosomes, all sex chromosomes ± the autosomal chromosome that has aneuploidy
Monosomy
The loss of a chromosome in an autosome as a result of non-disjunction
Trisomy
The gain of an extra chromosome in an autosome as a result of non-disjunction
Down’s Syndrome
47, XX or XY, +21
Trisomy 21
Intellectual disabilities and short stature
Turner Syndrome
45, XO
Short stature and sexual underdevelopment
Klinefelter Syndrome
47, XXY
Sexual immaturity