Genetics Flashcards
(24 cards)
Genetics
✓ Offspring acquire genes from parents by inheriting chromosomes
✓ Meiosis reduces the number of chromosome
✓ Genetic variation produced in sexual life cycles
✓ Mendel used the scientific approach to identify two laws of inheritance
Summary
- Mitosis and meiosis
- Somatic cells and gametes
- Main components of a chromosome
- Human chromosomes
The cells cycle
• A cell cycle is a series of events that takes place in a cell as it grows and divides.
• A cell spends most of its time in what is called interphase - it grows, replicates its chromosomes, and prepares for cell division. The cell then leaves interphase, undergoes mitosis, and completes its division.
• Resulting cells = daughter cells, each enter their own interphase and begin a new round of the cell cycle.
• Mitosis is a process of cell duplication, or reproduction, during which one cell gives rise to two genetically identical daughter cells.
• Mitosis is used for almost all of your body’s cell division needs.
• It adds new cells during development and replaces old and worn-out cells throughout your life
Problem + solution
• 23 pairs of chromosomes, division, 2 x 23 pairs needed = 23 for each daughter cell.
• DNA: Long and thread-like DNA in a non-dividing cell is called chromatin
• Doubled, coiled, short DNA in a dividing cell is called chromosome
• Consists of 2 parts: chromatid and centromere
Solution: Chromosomes duplicate before division
• Two identical “sister” chromatids
attached at an area in the middle
called a centromere
• When cells divide, “sister”
chromatids separate and 1 goes to
each new cell.
Main elements of the chromosome
• In preparation for cell division, DNA is replicated and the chromosomes condense.
• Each duplicated chromosome has two sister chromatids, which separate during cell division.
• The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached
Types
An autosome is one of the 22 numbered pairs of chromosomes
We have 23 pairs of chromosomes (total of 46 chromosomes)
Sex chromosomes - Thais is the X and Y chromosome.
- XX female, XY male
Mitosis
Interphase
Prophase
(Pro-metaphase)
Metaphase
Anaphase
Telophase
Interphase
In early prophase:
• chromosomes start to condense
• mitotic spindle begins to form
• organises the chromosomes and move them around during mitosis
In late prophase/prometaphase:
• chromosomes more condensed, very compact.
•The nuclear envelope breaks down, releasing the chromosomes.
•The mitotic spindle grows more, and some of the microtubules start to “capture” chromosomes.
In metaphase:
• All the chromosomes align at the metaphase plate (chromosomes line up).
•kinetochores of each chromosome attached to microtubules from opposite spindle poles.
•If not properly aligned or attached, the cell will halt division until the problem is fixed.
In anaphase:
sister chromatids separate from each other and are pulled towards opposite ends of the cell.
In telophase:
cell is nearly done dividing, starts to re-establish normal structures as cytokinesis takes place
Cytokinesis, the division of the cytoplasm to form two new cells, overlaps with the final stages of mitosis
Mitosis and Meiosis
• Meiosis is used for just one purpose in the human body: the production of gametes— sex cells, or sperm and eggs.
• Meiosis is a type of cell division that reduces the number of chromosomes in a parent cell by half to produce four reproductive cells.
• To put that another way, meiosis in humans is a division process that takes us from a diploid cell—one with two sets of chromosomes—to haploid cells—ones with a single set of chromosomes.
• In humans, the haploid cells made in meiosis are sperm and eggs.
• Meiosis is essential to reproduction, because it enables each parent to contribute one set of chromosomes – half the total – to each diploid offspring.
• In meiosis I homologous chromosomes pair up, and each pair separates, producing two haploid cells with their sister chromatids still joined.
• Meiosis II like mitosis; sister chromatids separate and four haploid cells are formed.
• Each cell has half the chromosomes of the parent cell. The gametes produced in meiosis aren’t genetically identical to the starting cell, and they also aren’t identical to one another.
Mitosis and meiosis 2
• Meiosis is essential to reproduction, because it enables eachparent to contribute one set of chromosomes – half the total – to each diploid offspring.
• In meiosis I homologous chromosomes pair up, and each pair separates, producing two haploid cells with their sister chromatids still joined.
• Meiosis II like mitosis; sister chromatids separate and four haploid cells are formed.
• Each cell has half the chromosomes of the parent cell. The gametes produced in meiosis aren’t genetically identical to the starting cell, and they also aren’t identical to one
another
Mechanisms of genetic variation
Radom fertilisation - genes Radom selected from pool provided by male and female gametes
Crossover
Random assortment of homologous chromosomes
Recombination - occurs during meiosis
Recombination is a process that breaks, recombines and rejoins sections of DNA to produce new combinations of genes.
Recombination during prophase I (meiosis), when Homology chromosomes line up in pair and swap segments of DNA.
This process, also known as crossing over, creates gametes that contain new combinations of genes, which helps maximise the genetic diversity of any offspring that results from the eventual union of two gametes during sexual reproduction.
Somatic cells and gametes
• Somatic cells (non-reproductive cells)
have two sets of chromosomes. Human
somatic cells (any cell other thana gamete)
have 23 pairs of chromosomes.
• Gametes (reproductive cells: sperm
and eggs) have half as many
chromosomes as somatic cells
An Intro to the genetics
• Heredity is the transmission of traits from one generation to the next
• Variation is demonstrated by the differences in appearance that offspring show from parents
• Genetics is the scientific study of heredity and variation
Genetic dictionary
- gene (unit of heredity)
- locus
- allele (variant form of gene)
- character
- trait
- phenotype x genotype
- homozygous x heterozygous
- dominant x recessive
Alleles
Alleles can be dominant or recessive.
• Dominant allele: if someone has two different alleles, dominant is the one that wins.
• It is indicated by a capital letter e.g. «P» for purple colour.
• Recessive allele: is the opposite of the dominant one. If someone has two different alleles, recessive in the one is the allele that loses.
• It is represented by the small letter e.g. «p» for white.
• The gene that governs wing length in fruit flies may be vestigial (vg) or normal (+)
Homozygous and heterozygous
• Homozygous: where both alleles in a pair of
homologous chromosomes are the same,
e.g.
P/P or p/p
B/B or b/b
• Heterozygous: where both alleles in a pair of
homologous chromosomes are different, e.g.
P/p
B/b
Homologous chromosomes
• Homologous chromosomes (“homologues”) are the same kind of chromosome with
the following elements:
• They look the same
• They carry genes for the same traits
• They may carry different alleles of these genes
• Only ONE of each homologous pair of chromosomes is in each haploid gamete.
Genotype vs Phenotype
• We can describe a person’s genes in two different ways.
• We can look at the person’s individual alleles and we call this the genotype.
• Genotype: the actual alleles present in an organism for any given trait
• Ex. TT, Tt, or tt in peas; vg/vg or vg/+ in fruit flies.
• We can also look at a person’s physical traits which we call the phenotype.
• Phenotype: the set of observable characteristics of an individual resulting from the interaction of its
genotype with the environmen
Character vs Trait
• Character is a heritable feature that varies among individuals
• Ex. Flower colour, eye colour, wing length
• Traits are the variants for a character
• Ex. Purple or white flower; green or grey eyes, vestigial or norm
Gregor Mendel
Mendel was a scientist, Augustinian friar and abbot of St. Thomas’ Abbey in Brno.
• He is considered the founder of the modern science of Genetics.
• Mendel’s pea plant experiments conducted between 1856 and 1863 established many of the rules of heredity, now referred to as the laws of Mendelian inheritance.
Mendel and Genes
Worked with seven characteristic of pea plants
The law of probability govern Mendelian inheritance
Inheritance is often more complex than simple Mendelian genetics
Many human traits follow Mendelian patterns of inheritance
Mendel’s Experimental, Quantitative Approach
• Advantages of pea plants for genetic study:
• There are many varieties with distinct heritable features, or characters (such as flower color)
• Character variants (such as purple or white flowers) are called traits
• Mating of plants can be controlled
• Each pea plant has sperm-producing organs (stamens) and egg- producing organs (carpels)
• Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another
Mendel’s laws
Mendel’s First Law - The Law of Segregation
• Resulted from experiments with only a single character
Mendel’s Second Law - The Law of Independent Assortment
• Resulted from experiments with two character at the same tim
Mendel’s First Law - The Law of Segregation
• Mendel chose to track only those characters that varied in two distinct, alternative forms, such as purple or white flower color.
Mendel started experiments with true-
breeding varieties:
• When plants self-pollinate, all
offspring are of the same variety
Mendel’s Peas
Hybridisation
- The mating or crossing between two individuals that have different characteristics - purple flowered plant X white flowered plant
Hybrids
- The offspring that results from such a mating
Mendel’s First Law - The Law of Segregation
• The true-breeding parents are the P generation.
• hybrid offspring of the P generation are called the F1 generation.
• When F1 individuals self-pollinate, the F2 generation is produced.
• When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple.
• When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white.
• Mendel discovered a ratio of about 3:1, purple to white flowers, in the F2 generation.
• Mendel developed an hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring.
• The law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
Mendel’s model
• Mendel developed a hypothesis to explain the 3:1 inheritance
pattern he observed in F2 offspring
• Four related concepts make up this model
• These concepts can be related to what we now know about
genes and chromosomes
• The First Concept is that alternative versions of genes account for variations in inherited characters
• For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers
• These alternative versions of a gene are now called alleles
• Each gene resides at a specific locus on a specific chromosome
• The Second Concept is that for each character an organism inherits two alleles, one from each parent
✓ Mendel made this deduction without knowing about the role of chromosomes
✓ The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation
✓ Alternatively, the two alleles at a locus may differ, as in the F1 hybrids
• The Third Concept is that if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has n noticeable effect on appearance
✓ In the flower-color example, the F1
plants had purple flowers because the allele for that trait is dominan
• The Fourth Concept, now known as the law of segregation, states that the two alleles for a heritable
character separate (segregate) during gamete
formation and end up in different gametes
• Thus, an egg or a sperm gets only one of the two
alleles that are present in the somatic cells of an organism
• This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis
