GENES AND HEREDITY

Students will recognize how biological traits are passed on to successive generations.

The Inheritance of Traits Most children are similar to their parents Children tend to be similar to siblings Each child is a combination of parental traits The combination of paternal traits and maternal traits is unique for each individual child

The Human Life Cycle gametes (a male sperm cell + a female egg cell) fuse during fertilization to form a single celled zygote, or embryo the embryo grows by cell division in mitosis the embryo grows into a child the child matures into an adult

Figure 6.1 The human life cycle. A human baby forms from the fusion of an egg cell from its mother and a sperm cell from its father. The single cell that results from this fusion will grow and divide into trillions of cells, each carrying the same information.

Deoxyribonucleic Acid All living things contain: Deoxyribonucleic Acid DNA Control the structure and function of cells When organisms reproduce, they pass on copies of their DNA to their offspring. Because offspring have DNA from their parents, they will resemble their parents.

DNA is sort of like an instruction manual that shows how to build and maintain a living organism…

Genes segments of DNA that carry information about how to make proteins Structural proteins – for things like hair Functional proteins – for things like breaking down lactose

Genes Are on Chromosomes located on the chromosomes number of chromosomes depends on the organism Bacteria – one circular chromosome Humans – 23 homologous pairs of linear chromosomes; one came from mom and the other from dad

Figure 6.3 The formation of different alleles. (a) Each parent provides a complete set of instructions to each offspring.

Passing traits from parents to offspring is called heredity.

Mutation Errors in gene copies can cause somewhat different proteins to be produced

Diversity in Offspring That means a unique egg will be fertilized by a unique sperm to produce a unique child For each gene, there is a 50% chance of having the same as a sibling

Diversity in Offspring The combination from the parents creates the individual traits of each child Environment also plays a role, but differing alleles from parents are the primary reason that non-twin siblings are not identical

Figure 6.6 Crossing over increases diversity in gametes. When chromosomes pair at the beginning of meiosis, information may be exchanged via the process of crossing over. (a) In our instruction manual analogy, this means that meiosis can result in the formation of completely new chromosome. (b) A photomicrograph of chromosomes in the process of crossing over during prophase of meiosis I.

Twins Fraternal (non-identical) two separate fertilized eggs not genetically the same

Figure 6.7 The formation of twins. (a) Dizygotic twins form when 2 different eggs combine with 2 different sperm cells, resulting in 2 embryos who are only as similar as siblings.

Twins Identical monozygotic: one single fertilized egg that separates genetically the same

Figure 6.7 The formation of twins. (b) Monozygotic twins form when a newly fertilized embryo splits in half resulting in 2 identical embryos.

Gregor Mendel Determined how traits were inherited Used pea plants and analyzed traits of parents and offspring Figure E6.1a Gregor Mendel

Genotype Genotype homozygous: two of the same heterozygous: two different

Phenotype Phenotype the physical outcome of the genotype depends on nature of genes

Mendelian Genetics Dominant – can mask a recessive Recessive – can be masked by a dominant gene

Mendelian Genetics Dominant – capital letter For example: T for tall Recessive – lower case letter For example: t for short

Punnett Squares Punnett squares are used to predict offspring phenotypes Uses possible gametes from parents to predict possible offspring

Punnett Squares: Single Gene A parent who is heterozygous for a trait Aa can produce two possible gametes A or a A parent who is homozygous for a trait AA can only produce gametes with A

Punnett Squares The possible alleles are listed along the top and side of the square The predicted offspring genotypes are filled in the center boxes of the square

Punnett Squares The offspring can be homozygous or heterozygous It all depends on the parents and the possible gametes Punnet squares can be used to predict possibilities of inheriting genetic diseases

The environment plays a role – traits such as height, weight, musical ability, susceptibility to cancer, and intelligence

Environment can affect phenotypes Identical twins with the same genotypes may not have exactly the same appearance…

Figure 6.14 The effect of the environment on phenotype. These identical twins have exactly the same genotype, but they are quite different in appearance due to environmental factors.

Why Traits Are Quantitative Figure 6.15 Skin color is influenced by genes and environment. (a) The difference in skin color between these two women is due primarily to variations in several alleles that control skin pigment production. (b) The difference in color between the sun-protected and sun-exposed portions of the individual in this picture is entirely due to environmental effects.

Correlation between Twins Twin studies allow scientists to test the effects of environment The DNA is identical in identical twins but the environment may be different Compare monozygotic (identical) twins to dizygotic (fraternal) twins Study twins raised together and study identical twins raised apart