Inheritance of genes

Autosomes

(non-sex chromosomes)

Dominant Allele and Dominant Trait.

Although people with a Pp genotype have one PKU allele (p), the allele that codes for the normal trait (P) masks the presence of the PKU allele. An allele that dominates or masks the presence of another allele and is fully expressed (P in this example) is said to be a dominant allele, and the trait expressed is called a dominant trait.

Somatic cell

Any cell that isn't a gamete ---- contains two copies of every allele, one obtained from each biological parent.

Common inheritance patterns include:

Complete dominance, in-complete dominance, co-dominance, and sex-linked (X-linked) traits.

Histones

DNA is wrapped around proteins called histones to prevent tangling.

Allele

Each member of a gene pair (each copy of the gene) is known as an allele.

Allele

Each member of the gene pair is called an allele, and the members can be identical to each other or slightly different. An allele is an alternative form of a gene. It is the differences in alleles that give rise to different genotypes. Alleles may have differences in the sequences of only one or a few DNA base pairs, but that small difference means they produce different proteins than their counterpart genes.

Where are genes found? and do they come in pairs?

Genes are found in specific locations on their chromosomes. We know that chromosomes come in pairs, which means genes come in pairs as well.

Genes

Genes are located on chromosomes and come in pairs in most human cells.

Chromosomes

Genes are located on chromosomes. Human beings have 23 pairs of chromosomes (46 total) and these are conveniently numbered 1 - 22, for the non-sex chromosomes. The chromosomes that make up the last pair are called X and y. These are the sex chromosomes. In mammals, females have two X chromosomes and males have an X and a y chromosome. We inherit one chromosome of each pair from our biological father and one from our biological mother. The gametes (egg and sperm cells) contributed by each biological parent have only 23 chromosomes, one from each pair, and the chromosomes from both the egg and sperm together provide the full set of 46 chromosomes to their offspring.

The branch of biology that studies inheritance is called:

Genetics.

Fact

If our genes are a way that DNA organizes itself, then our chromosomes are our genes' way of organizing themselves. As discussed in the chapter "Genes to Proteins" above, genes are sections of DNA that code for a protein and are expressed (transcribed and translated) depending on the needs of the cell. However, each human cell has over 3 billion base pairs of DNA and ~20,000 genes!

Heterozygous genotype.

If the two alleles code for two variations of that trait the genotype is heterozygous.

Genotype.

Scientists refer to the exact sequence of nucleotides present on each individual's chromosomes as their genotype. We differ from each other in genotype by less than 1%, but that less than 1% makes all the difference in the world when discussing individuals

DNA forms chromosomes.

The DNA double helix is wrapped around histone proteins to form a nucleosome. Nucleosomes can form open chromatin, in which the nucleosomes look like beads on a string, and they can be condensed into a highly compacted chromatin fiber. This chromatin fiber is further folded to form the chromosome.

Recessive allele and Recessive trait.

The allele, whose presence is completely masked (p in this example), is said to be a recessive allele, and the trait it controls is called a recessive trait.

Genetic Counseling.

The area of health care that offers advice on genetic problems (or potential problems).

Chromosomes

The chromosomes are highly compact DNA, and they also serve as a kind of multilevel filing system for the genes--specific genes are located at specific sites on particular chromosomes (the gene for hemoglobin is always found on chromosome 11). This allows the cell to easily locate the genes it needs at any given time.

Humans have 23 pairs of chromosomes.

The chromosomes inherited from the mother are shown in blue and those inherited from the father are shown in red. Each chromosome in a pair of non-sex chromosomes is the same size because they carry the same amount and type of genetic information. This is not the case for the sex chromosomes. The X chromosome is much larger than the y chromosome, and they each carry different numbers (and types) of genes that confer a person's biological sex. Both an XX and an Xy pair are shown in the figure above for comparison, although an individual usually only has one pair of sex chromosomes.

Nucleosomes.

The combination of DNA and histone proteins

Genome facts:

The genome is divided among the 23 chromosomes and each chromosome carries a specific subset of those genes. Thus, a person has two copies of each gene, one on the chromosome inherited from the mother and one on the chromosome inherited from the father

Genotype

The genotype (pair of genes) decide the phenotype (observable characteristics) of an individual.

What do histones do?

The histones promote coiling of the nucelosomes into a larger chromatin fiber. The chromatin fibers are folded into large loops that eventually form chromosomes.

Genome.

The whole set of genes that a person has.

Punnett Squares Show Possible Inheritance Patterns.

This Punnett square shows the likely inheritance patterns of two parents who are heterozygous for PKU. The two possible sperm from the father are shown on the left side of the Punnett square and the two possible eggs from the mother are shown along the top of the square. The four square show the different possible combinations of the egg and sperm. Notice that there are two ways to have a heterozygous child, which makes the probability of the couple having a heterozygous child 50%. The two homozygous possibilities have a probability of 25% each. Since PKU is a recessive disease, the probability that a child will have the PKU phenotype is 25% - the same probability of a homozygous recessive genotype

The very small difference (less than 1%) in genotype between you and your neighbor is because of what?

This is because slightly different alleles, which combine to give rise to some very different phenotypes.

Inheritance

This is the passage of hereditary traits from one generation to the next. It is the process by which you acquired your characteristics from your biological parents and may transmit some of your traits to your children.

Representing the Inheritance of Alleles

Visualizing and predicting how different alleles will be inherited based on the genetics of the parents is an important aspect of understanding inheritance. The relationship of genes to heredity is illustrated by examining the alleles involved in a disorder called phenylketonuria (PKU). People with PKU are unable to manufacture the enzyme phenylalanine hydroxylase and are therefore unable to metabolize the amino acid phenylalanine. The normal allele that codes for phenylalanine hydroxylase is symbolized as P; the mutated allele that fails to produce a functional enzyme is represented by p.

Phenotype and Genotype.

We all have the same basic arrangement of genes in our chromosomes, despite individual differences in phenotype. The phenotype is all the observable characteristics or traits of an individual, including ones that are not easily seen, such as blood type or color blindness. These phenotypic differences emerge from subtle differences in genotype--our complete set of genes--as well as environmental factors. ----- Our phenotype is the result of our genotype and all the environmental influences on us, including the quality of our food, the type of shelter we live in, and even our financial security.

Homozygous genotype

When the two alleles are identical, the genotype is homozygous for the trait that is controlled by those alleles.