Biology 6.4 Traits, Genes, & Alleles

Main Ideas

✔The same gene can have many variations. ✔Genes influence the developement of traits.

Dominant & Recessive Alleles ⬇⬇⬇⬇⬇⬇

✔DOMINANT allele is expressed when 2 different alleles are present. ✔RECESSIVE allele is expressed only when 2 copies are present.

Alleles & Phenotype

Because some alleles are dominant over others, 2 genotypes can produce the dominant phenotype. Smooth pods & constricted pods in pea plants. A plant with smooth pods could have a homozygous dominant (SS) or heterozygous (Ss) genotype. In contrast, a plant with constricted, or compressed, pods could only have a homozygous recessive (ss) genotype. What actually makes one allele dominant over another? The answer is very complicated. It depends on the nature of the protein that is, or is not, made. Pigment gives cell color. If P directs flower cells to make pigment, the flower may look purple. If p directs the cells not to make pigment, the flower looks white. So P codes for pigment to be present, but p codes for nothing, the absence of pigment. As a result, P has to be dominant. Even if the flower has only 1 P allele (Pp), that 1 allele tells its cells to make pigmenf, & the flower has color. Flower pigment is only one example. Many factors make one allele dominant over another. Most plants are not simply small or short. Most flowers are not just white or purple. Mosylt states occur in a range. Other factors also affect traits. A lack of sunshine or vital nutrients could stunt a plant's growth. How does genetics account for these issues? Mendel studied traits that follow simple dominant-recessive patterns of inheritance, & each trait was the result of a single gene. In general, however, inheritance is much more complex. Most alleles are not simply dominant or recessive; some are codominant. Many traits are influenced by multiple genes. The environment also interacts with genes and affects their expression.

Key Concept

Genes encode proteins that produce a diverse range of traits.

Genes influence the developement of traits

Human Genome Project's goal was to find out the sequence of the 3 billion nucleotide pairs that make up a human's genome. A GENOME is all of an organism's genetic material. Unless you have an identical twin, you have a unique genome that determines all of your traits. Some of your traits can be seen, such as the color of your eyes. Other traits cannot be seen, such as the color of your eyes. Other traits cannot be seen, such as the exact chemical makeup of your eyeball. In genetics, we often focus on a single trait or set of traits. A genome is all of an organism's genes, but a GENOTYPE typically refers to the genetic makeup of a specific set of genes. The genotype of a pea plant includes both of the genes that code for flower color, even if one of those genes is masked. In contrast, the physical characteristics, or traits, of an individual organism make up its PHENOTYPE. A pea plant with purple flowers has a phenotype for purple flowers. The plant might have a hidden gene for white flowers, but that does not matter to its phenotype.

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If an organism is a heterozygous for a trait, which allele will be expressed? That is, if a plant has 1 allele for purple flowers and one for white flowers, what color will the flowers be? As Mendel learned, one allele that is expressed when 2 different alleles are present. A RECESSIVE allele is the allele that is only expressed when 2 copies are present. In Mendel's experiments, the allele for purple flowers was dominant to the allele for white flowers. All F1 plants were purple even though they had only one allele for purple flowers. Sometimes the word dominant is misunderstood. A dominant allele in not necessarily better or stronger than a recessive allele. It does not necessarily occur most often in the population. An allele is dominant in a heterozygote simply because it is expressed and the other allele is not. Alleles are often represented on paper with individual letters. An organism's genotype for a trait can be shown with 2 letters-1 per allele. Uppercase letters are used for dominant alleles, lowercase letters are used for recessive alleles. For example, the dominant allele for height in pea plants is written as T, for tall. The recessive allele for short plants is written as t. A plant's genotype might be homozygous dominant (TT), heterozygous (Tt), or homozygous recessive (tt).

The same gene can have many versions

Mendel's discrete units of heredity are now called genes. But what are genes? You can think of a GENE as a piece of DNA that provides a set of instructions to a cell to make a certain protein. This definition is not precise, but it gives you the main idea. Each gene has a locus, a specific position on a pair of homologous chromosomes. Just as a hosue is a physical structure and an address tells where that house is located, you can think of the locus as the "address" that tells where a gene is located on a chromosome. Most genes exist in many forms. In Mendel's experiments, the effects of these different forms were easy to see: yellow or green, round or wrinkled. An ALLELE is any of the alternative forms of a gene that may occur at a specific locus. Your cells have 2 alleles for each gene, one on each of the homologous chromosomes on which the locus for that gene is found. Each parent gives one allele. The 2 alleles may be the same, or they may be different. The term HOMOZYGOUS describes two of the same alleles at a specific locus. For example, both might code for white flowers. The term HETEROZYGOUS describes 2 different alleles at a specific locus. Thus, one might code for white flowers, the other for purple flowers. Homo= same Hetero= different

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Polydactyly is the condition of having more than the typical number of fingers or toes. The allele for polydactyly is dominant.