Haploids Versus Diploids

• A cell that has two sets of chromosomes is a diploid cell.

• The chromosome number is given as “2n.” That means we have two copies of each chromosome.

• If a cell has only one set of chromosomes, we call it a haploid cell. This kind of cell is given the symbol n.

• The duplicate versions of each chromosome are called homologous chromosomes.

• The homologous chromosomes that make up each pair are similar in size and shape and contain the same genes in the same locations

Gametes

• Sex cells are haploid.

• A parent will contribute a gamete with one set that will be paired with the set from the other parent to produce a new diploid cell, or zygote.

Gregor Mendel: The Father of Genetics

• Genetics was discovered by the monk Gregor Mendel

• Traits are influenced by one or more of your genes.

• The position of a gene on a chromosome is called a locus.

• Diploid organisms usually have two copies of each gene, one on each homologous chromosome.

• Homologous chromosomes are two copies or versions of the same chromosome in a diploid cell or organism.

• Humans have 23 pairs of homologous chromosomes.

• Homologous chromosomes are the same size and shape, and contain the same genes. However, they can contain different versions(alleles) of those genes, and thus have different genetic sequences.

• When an organism has two identical alleles for a given trait, the organism is homozygous.

• If an organism has two different alleles for a given trait, the organism is heterozygous.

• When discussing the physical appearance of an organism, we refer to its phenotype.

• The genotype tells us which alleles the organism possesses.

• The dominant allele receives a capital letter and the recessive allele receives a lowercase of the same letter.

• Label each generation in the cross.

  • The first generation in an experiment is always called the parent, or P generation.

  • The offspring of the P generation are called the first filial, or F1 generation. Members of the next generation, the grandchildren, are called the F2 generation.

• The three principles of genetics: the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment.

The Law of Dominance

• Mendel crossed two true-breeding plants with contrasting traits: tall pea plants and short pea plants.

• To his surprise, when Mendel mated these plants, the characteristics didn’t blend to produce plants of average height. Instead, all the offspring were tall.

• A monohybrid cross occurs when two individuals are crossed and one gene is being studied. A simple way to represent a monohybrid cross is to set up a Punnett square. Punnett squares are used to predict the results of a cross.

The Law of Segregation

Next, Mendel took the offspring and self-pollinated them.

• Here’s a summary of the results: The ratio of phenotypes is 3:1 (three tall:one short). The ratio of genotypes is 1:2:1 (one TT:two Tt:one tt).

The Law of Independent Assortment

• So far, we have looked at only one trait: tall versus short.

• What happens when we study two traits at the same time? Each allele of the two traits will get segregated into two gametes, but how one trait gets split up into gametes has no bearing on how the other trait gets split up.

Dihybrid Cross

• Different genes assort independently into gametes. A dihybrid cross is just like the monohybrid, but it studies how two genes are passed on to offspring.

Rules of Probability

• A better method for predicting the likelihood of certain results from a dihybrid cross is to apply the Rules of Probability.

• To determine the probability that two or more independent events will occur simultaneously, one can simply calculate the product of the probability that each will occur independently. This is called the Product Rule.

• To determine the likelihood that EITHER event occurs, but not both, use another rule called the Sum Rule.

• Product Rule: If A and B are independent, then: P(A and B) = P(A) times P(B) Sum Rule: If A and B are mutually exclusive, then: P(A or B) = P(A) + P(B)

• To find the probability of having a tall, yellow plant, simply multiply the probabilities of each event. If the probability of being tall is 3/4 and the probability of being yellow is 1/4, then the probability of being tall and yellow is 3/16 .

SUMMARY OF MENDEL’S LAWS

• Law of Dominance: One trait masks the effects of another trait.

• Law of Segregation: Each gamete gets only one of the copies of each gene.

• Law of Independent Assortment: Each pair of homologous chromosomes splits independently, so the alleles of different genes can mix and match.

• Suppose we want to know if a tall plant is homozygous (TT) or heterozygous (Tt). Its physical appearance doesn’t necessarily tell us about its genetic makeup. The only way to determine its genotype is to cross the plant with a recessive, short plant, tt. This is known as a test cross.

Non-Mendelian Genetics Sex-Linked Traits

Linked Genes

• Sometimes genes on the same chromosome stay together during assortment and move as a group.

• The group of genes is considered linked and tends to be inherited together. For example, the genes for flower color and pollen shape are linked on the same chromosomes and show up together.

• Since linked genes are found on the same chromosome, they cannot segregate independently.

• This violates the Law of Independent Assortment.

• The offspring formed from recombination events are called recombinants.

• The percentage of recombination (recombination frequency) can be determined by adding up the recombinants and dividing by the total number of offspring

• The frequency of crossing-over between any two linked alleles is proportional to the distance between them. This finding led to recombination mapping— mapping of linkage groups with each map unit being equal to 1 percent recombination.

• For example, if two linked genes, A and B, recombine with a frequency of 15 percent, and B and C recombine with a frequency of 9 percent, and A and C recombine with a frequency of 24 percent, what is the sequence and the distance between them?

• A-B is 15 units

• B-C is 9 units

• Total 24 units

Sex-linked traits

• Humans contain 23 pairs of chromosomes. Twenty- two of the pairs of chromosomes are called autosomes.

• They code for many different traits.

• The other pair contains the sex chromosomes. This pair determines the sex of an individual.

• A female has two X chromosomes. A male has one X and one Y chromosome.

• Some traits, such as color blindness and hemophilia, are carried on sex chromosomes.These are called sex-linked traits.

• Most sex- linked traits are found on the X chromosome and are more properly referred to as “X-linked.”

• Since males have one X and one Y chromosome, what happens if a male has an X-chromosome with the color blindness allele? Unfortunately, he’ll express the sex-linked trait, even if it is recessive.

• However, if a female has only one color blind-X chromosome, she won’t express a recessive sex-linked trait. For her to express the trait, she has to inherit two color blind-X chromosomes.

• A female with one color blind-X is called a carrier. Although she does not exhibit the trait, she can still pass it on to her children.

• You can also use the Punnett square to figure out the results of sex-linked traits.

Barr Bodies

• A look at the cell nucleus of normal females will reveal a dark-staining body known as a Barr body.

• A Barr body is an X chromosome that is condensed and visible. In every female cell, one X chromosome is activated and the other X chromosome is deactivated during embryonic development.

• The X chromosome destined to be inactivated is randomly chosen in each cell.

• Therefore, in every tissue in the adult female, one X chromosome remains condensed and inactive. However, this X chromosome is replicated and passed on to a daughter cell.

• X-inactivation is the reason it is okay that females have two X chromosomes and males have only one. After X-inactivation, it is like everyone has one copy.

Other Inheritance Patterns

• Incomplete dominance (**blending inheritance): In some cases, the traits will blend. For example, if you cross a white snapdragon plant (genotype WW) with a red snapdragon plant (RR), the resulting progeny will be pink (RW). In other words, neither color is dominant over the other.

• Codominance: Sometimes you’ll see an equal expression of both alleles. For example, an individual can have an AB blood type. In this case, each allele is equally expressed.

• Polygenic inheritance: In some cases, a trait results from the interaction of many genes. Each gene will have a small effect on a particular trait.

• Non-nuclear inheritance: Apart from the genetic material held in the nucleus, there is also genetic material in the mitochondria. The mitochondria are always provided by the egg during sexual reproduction, so mitochondrial inheritance is always through the maternal line, not the male line. In plants the mitochondria are provided by the ovule and are maternally inherited.

Pedigrees

• One way to study genetic inheritance is by looking at a special family tree called a pedigree.

• A pedigree shows which family members have a particular trait and it can help determine if a trait is recessive or dominant and if it is sex-linked.

• Traits that skip generations are usually recessive.

• Traits that appear more in one sex than the other are usually sex-linked.

• In a pedigree chart, the males are squares and the females are circles.

Environmental Effect on Traits

• Changes in genotypes can result in changes in phenotype, but environmental factors also influence many traits, directly and indirectly.

• Furthermore, an organism’s adaptation to the local environment reflects a flexible response of its genome

• Phenotypic plasticity occurs if two individuals with the same genotype have different phenotypes since they are in different environments.

An Overview of Meiosis

• Meiosis is the production of gametes.

• Meiosis is limited to sex cells in special sex organs called gonads.

• In males, the gonads are the testes, while in females they are the ovaries.

• The special cells in these organs—also known as germ cells—produce haploid cells (n), and they combine to restore the diploid (2n) number during fertilization. female gamete (n) + male gamete (n) = zygote (2n)

Meiosis is likely to produce sorts of variations than is mitosis, which therefore confers selective advantage on sexually reproducing organisms.

A Closer Look at Meiosis

• Meiosis actually involves two rounds of cell division: meiosis I and meiosis II.

• Before meiosis begins, the diploid cell goes through interphase. Just as in mitosis, double-stranded chromosomes are formed during S phase.

• Meiosis I

  • Meiosis I consists of four stages: prophase I, metaphase I, anaphase I, and telophase I.

  • Meiosis I ensures that each gamete receives a haploid (1n) set of chromosomes.

• Prophase I

  • As in mitosis, the nuclear membrane disappears, the chromosomes become visible, and the centrioles move to opposite poles of the nucleus.

  • The major difference involves the movement of the chromosomes. In meiosis, the chromosomes line up side-by-side with their counterparts (homologs). This event is known as synapsis.

  • Synapsis involves two sets of chromosomes that come together to form a tetrad (a bivalent). A tetrad consists of four chromatids. Synapsis is followed by crossing-over, the exchange of segments between homologous chromosomes.

  • What’s unique in prophase I is that pieces of chromosomes are exchanged between homologous partners. This is one of the ways organisms produce genetic variation.

• Metaphase I

  • As in mitosis, the chromosome pairs—now called tetrads—line up at the metaphase plate.

  • By contrast, you’ll recall that in regular metaphase, the chromosomes line up individually.

  • One important concept to note is that the alignment during metaphase is random, so the copy of each chromosome that ends up in a daughter cell is random.

• Anaphase I

  • During anaphase I, each pair of chromatids within a tetrad moves to opposite poles. The homologs will separate with their centromeres intact.

  • The chromosomes now move to their respective poles.

• Telophase I

• During telophase I, the nuclear membrane forms around each set of chromosomes.

• Finally, the cells undergo cytokinesis, leaving us with two daughter cells.

• Meiosis II

  • The purpose of the second meiotic division is to separate sister chromatids

• During prophase II, chromosomes once again condense and become visible.

• In metaphase II, chromosomes move toward the metaphase plate. This time they line up single file, not as pairs.

• During anaphase II, chromatids of each chromosome split at the centromere, and each chromatid is pulled to opposite ends of the cell.

• At telophase II, a nuclear membrane forms around each set of chromosomes and a total of four haploid cells are produced.

Gametogenesis

• Meiosis is also known as gametogenesis.

• If sperm cells are produced, then meiosis is called spermatogenesis.

• During spermatogenesis, four sperm cells are produced for each diploid cell.

• If an egg cell or an ovum is produced, this process is called oogenesis.

• Oogenesis produces only one ovum, not four. The other three cells, called polar bodies, get only a tiny amount of cytoplasm and eventually degenerate since the female wants to conserve as much cytoplasm as possible for the surviving gamete, the ovum.

Meiotic Errors

• Nondisjunction—chromosomes failed to separate properly during meiosis.

• This error, which produces the wrong number of chromosomes in a cell, usually results in miscarriage or significant genetic defects.

• Individuals with Down syndrome have three—instead of two—copies of the 21st chromosome.

• Nondisjunction can occur in **anaphase I (**meaning chromosomes don’t separate when they should), or in anaphase II (meaning chromatids don’t separate).

• Either one can lead to aneuploidy, or the presence of an abnormal number of chromosomes.