Just the facts

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Concept 1: Sexually reproducing organisms make sex cells or gametes.

Gametes serve two functions. They have the physiological capability to fuse with a gamete from the opposite sex and form a single cell (a zygote) that initiates the development of a new organism and the next generation. They also carry a complete set of genetic instructions that this new organism will need to grow, develop and complete its life cycle. Gametes are made from the cells of an organism through a special division process called meiosis (see other lesson). This cell division process allows the gametes to have half of the genetic material of the original cell. The genetic material is packaged in structures called chromosomes and the chromosomes sorted in an orderly fashion to give each gamete the one set of genetic instructions. Offspring made from sexual reproduction will have two sets of genetic instructions, one delivered from the male gamete (pollen in plants, sperm in animals) and one from the female egg. Fig 1. Somatic cells have two of each type of chromosome, gametes have one of each. Fig.2: Sex cells are made in specialized organs and carry genetic information to the next generation.

Concept 2: Genes are stable, passed on in sex cells and control traits

We now know that genes are segments of DNA, the deoxyribonucleic acid molecule that makes up chromosomes. DNA is a stable molecule and therefore genes are stable. They maintain their structural integrity as they are copied and passed on from cell to cell and from generation to generation. Because they maintain their structure, genes can reliably encode genetic information, instruct the cell how to make specific proteins and ultimately control traits or characteristics in the organism. Geneticists use the term phenotype to designate the trait or combination of traits observed in an individual. Fig. 3: DNA, deoxyribonucleic acid, has subunits that code information and assemble into a stable double helix structure. Fig. 4: A chromosome is a long DNA molecule and contains many genes, each with it's own coded information. Fig. 5: The coded information in genes instructs the cell to make proteins that control specific traits or phenotypes such as the color of cells and tissues.

Concept 3: Genes are a part of the chromosome and are found in pairs in somatic cells.

The idea that the genes which control a specific trait are found in pairs in somatic cells was proposed by a geneticist who never saw a gene. Gregor Mendel didn’t even know about chromosomes when he proposed this idea. He observed a pattern of inheritance in pea plants that could be explained by this idea. Now we know that genes are in pairs in somatic cells because the chromosomes are in pairs. A cell will have two of each kind of chromosomes (with the exception of sex chromosomes in one of the sexes) and therefore two of each of the genes found on those chromosomes. Fig. 6: Genes are in pairs in somatic cells because chromosomes are in pairs. The paired chromosomes seperate during gametes formation.

Concept 4: Genes can change on occasion and the alternative version or allele may control the trait

While genes are stable, they can be altered to encode different genetic information. An alternative form of a gene is called an allele. Genetic variation relies upon alleles. To designate genes and their alleles we often use single letters such as ‘A’ and ‘a’. Fig. 7: Changing the coding region of a gene will result in the production of a different protein. This can alter the organism's traits.

Concept 5: Individuals can be homozygous or heterozygous.

Homo means same and hetero means different. Zygous stands for zygote. If genes are in pairs in somatic cells and genes can have alleles, different genetic combinations can occur. An individual can have two copies of the same allele (homozygotes AA or aa) or they can have two different alleles (heterozygote Aa). The genetic makeup of an individual at specific gene pairs is called the genotype. Fig. 8: A somatic cell from an individual who is heterozygous at the B,b gene pair and homozygous at the A gene pair..

Concept 6: Gene pairs associate and then separate during gamete formation

This idea is essentially Mendel’s first law of genetics, the Principle of Segregation. Again, it is remarkable that Mendel proposed the idea from inference and not from direct observation of gene pairs separating during meiosis. Now we know that gene pairs associate and separate because genes are a part of chromosomes and chromosomes pair and separate during meiosis. Fig. 9: The principle of segregation is the idea that paired genes seperate when gametes are formed.

Concept 7: Gametes combine at random to form the individuals in the next generation.

Anyone who has been in a corn field during sexual reproduction has witnessed the random nature of male and female gametes combining. Once the pollen or egg is made, it is simply chance that dictates which gametes will get together. If a large sample of offspring from a given cross is examined, the genetic combinations will reflect the random nature of gametes combining and forming offspring. Fig. 10: Gametes combine at random with respect to the genes that they carry.

Concept 8: One allele can be dominant over another or show a lack of dominance.

The idea of dominance was first proposed by Mendel. He observed that when crossing two lines of peas that expressed alternative versions of a trait, the first generation of offspring (first filial or F1) all had the same phenotype. Mendel proposed the F1s were heterozygous. They had an allele from one parent (‘A’) that was dominant and masked the presence of the allele from the second parent (‘a’). The F1 was ‘Aa’, had a different genotype than the AA parent but the same phenotype. Mendel was convinced the F1 still held the ‘a’ or recessive allele because if the F1 was selfed, some of the F2 offspring expressed the recessive trait again. In other experiments performed by other geneticists, two alleles may have a lack of dominance. In this case, an F1 would be heterozygous but have a phenotype that was unique or intermediate to either of the parent line phenotypes. Fig. 11: The disease resistance allele is dominant over the disease lesion allele . Fig. 12: The green leaf color trait has a lack of dominance over yellow leaf color giving an 'in between' phenotype.

Concept 9: The segregation of genes allows geneticists to make predictions.

Geneticists like to predict the future. Genes behave in predictable ways which makes it possible to predict outcomes from a given cross. The favorite tool of the geneticist in making predictions is the Punnet square. The diagram depicts the process of segregation. The single letters on the outside of the squares represent the genes in the gametes of the parent. The paired letters in the squares show how the gametes can randomly come together and form the zygote and thus the next generation. Phenotype and genotype ratios can be determined from the squares in the diagram Fig. 13: The inheritance of genes is predictable behavior and therefore, trait inheritance can be predicted in controlled crosses.

Concept 10: Segregation can be predicted based on independent assortment.