Chapter 11 Genetics

Genetics All living things have a set of characteristics inherited from its parent or parents Genetics – the study of heredity Trait – a specific characteristic, such as seed color Genes – chemical factors (DNA segments) that control traits. Usually code for proteins Alleles – forms of a gene, like T (tall) or t (short)

Gregor Mendel Mendel was a monk in charge of the garden monastery ◦ Mendel had stocks of plants that produced only tall, only short, only green seeds, and only yellow seeds ◦ He studies cross-bred plants with different characteristics and studied the results ◦ The offspring (F1) of crosses between parents (P) with different traits are called hybrids

Gregor Mendel

All of the F1 generation had characteristic of only one parent (no blending) ◦ Each characteristic was controlled by 1 gene with two contrasting forms, called alleles ◦ Principle of dominance states that some alleles are dominant while others are recessive

Gregor Mendel

He then crossed F1 plants to produce F2 plants ◦ Recessive traits reappeared, roughly ¼ had the recessive traits ◦ The two alleles of the F1 plant segregate from each other so that a gamete only has 1 copy of the gene ◦ Each F1 plant gamete has either the dominant or recessive (tall or short)

Gregor Mendel

Mendel Gregor Mendel concluded: ◦ Inheritance is controlled by factors passed from one generation to the next. ◦ Some alleles are dominant and others are recessive. ◦ Segregation - the alleles (separate) during the formation of gametes. ◦ Independent assortment - genes for different traits can segregate independently during the formation of gametes.

Punnett Squares Punnett Squares – tool used to predict the probability of offspring with a certain trait or genotype. Monohybrid cross – Punnett square with one trait, 4 squares.

Genetic Terms Dominant – occurs in the phenotype if one allele is present, T Recessive – must be homozygous to occur in the phenotype Homozygous – having two identical alleles for a trait, TT or tt Heterozygous – having two different alleles for a trait, Tt Phenotype – physical characteristic, tall Genotype – genetic makeup, TT or Tt

Monohybrid Cross

Dihybrid Cross Since alleles can segregate independently, the inheritance of one trait doesn’t affect the inheritance of another (seed color and seed shape) A dihybrid cross involves crossing two different traits of the F1 generation (both parents are heterozygous for both traits)

Dihybrid Cross

Complex Forms of Inheritance Some alleles are neither dominant or recessive, and many traits are controlled by multiple alleles or multiple genes Codominance – both alleles are present in the phenotype, the black and white chicken, blood types Incomplete dominance – the alleles blend to produce the phenotype, red + white = pink

Blood Types: Codominance

Incomplete dominance

Complex Forms of Inheritance Multiple alleles – more than 2 alleles are possible choices, although only 2 can be used at a time, ex. ABO blood groups, rabbit coat color Polygenic traits – controlled by 2 or more genes, ex. skin color Sometimes expression is also affected by the environment; height and flower color of a sunflower are affected by genes and climate, soil, and water availability

Multiple Alleles

Sex-linked Genes that are located on sex chromosomes are said to be sex-linked genes ◦ Since males only have 1 X chromosome, all X- linked alleles are expressed, even if recessive ◦ For females, a recessive allele must be with another recessive to be expressed ◦ Colorblindness and hemophilia is an example

Sex-linked

Chapter 14 Heredity

Human Heredity A karyotype is a picture of chromosomes arranged in their pairs (46, 23 pairs in humans) MaleFemale

Human Heredity All egg cells carry an X chromosome, half sperm cells carry an X, half a Y ◦ Leads to half combined becoming either XY, or XX Autosomes are non-sex chromosomes, labeled pairs 1-22 chromosomes Sex chromosomes are the 23 rd pair of chromosomes

Human Heredity To study how traits are passed from generation to generation a pedigree chart is used ◦ Males are squares ◦ Females are circles ◦ Shaded circle/squares have the trait ◦ Horizontal lines represent marriage ◦ Vertical lines connect parents to their offspring White forelock (lock of hair just above the forehead) is dominant

Human Heredity

Many disorders are caused by an autosomal recessive allele (disease is only apparent in the homozygous recessive condition) Some disorders are expressed with only one allele (dominant), which means the disease is apparent in the heterozygous condition Some disorders are caused by a codominant alleles

Human Heredity Disorder(s)Inheritance patternSymptoms Cystic fibrosisSimple (autosomal) recessive Mucus in lungs, digestive tract, liver Sickle cell diseaseSimple (autosomal) recessive (codominant on molecular level) Sickle shaped red blood cells; joint pain, anemia Huntington’s disease Simple (autosomal) dominant Mental deterioration and uncontrollable movements; onset Colorblindness Sex-linked recessiveDifficulty discerning colors HemophiliaSex-linked recessiveBlood doesn’t clot correctly, missing Factor VIII Down syndrome (Trisomy 21) Trisomy (3 chromosomes instead of 2) Mild to severe mental retardation

Human Heredity Cystic fibrosis leads to serious digestive problems ◦ Thick, heavy mucus clogs lungs and breathing passages ◦ Since it is recessive, must have 2 alleles to be affected Sickle cell disease leads to bent/twisted shape of RBCs (can clog in capillaries) ◦ Heterozygous individuals are generally healthy and more resistant to malaria

Chromosomal Disorders Down syndrome (trisomy 21) ◦ Nondisjunction occurs when homologous chromosomes don’t separate during meiosis- leads to abnormal number of chromosomes ◦ 3 copies of chromosome 21 ◦ Mild to severe mental disability, and higher susceptibility to many diseases Sex chromosome nondisjunction ◦ Turner syndrome (X): sterile ◦ Klinefelter’s syndrome (XXY): generally sterile