Chapter 11 Genetics
Chapter 11Genetics
GeneticsAll living things have a set of
characteristics inherited from its parent or parents
Genetics – the study of heredityTrait – a specific characteristic, such as
seed colorGenes – chemical factors (DNA
segments) that control traits. Usually code for proteins
Alleles – forms of a gene, like T (tall) or t (short)
Gregor MendelMendel 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
Gregor MendelAll 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
Gregor MendelHe 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
MendelGregor 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 SquaresPunnett 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 TermsDominant – occurs in the phenotype if
one allele is present, TRecessive – must be homozygous to
occur in the phenotypeHomozygous – having two identical
alleles for a trait, TT or ttHeterozygous – having two different
alleles for a trait, TtPhenotype – physical characteristic,
tallGenotype – genetic makeup, TT or Tt
Monohybrid Cross
Dihybrid CrossSince 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 InheritanceSome 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 InheritanceMultiple 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-linkedGenes 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 14Heredity
Human HeredityA karyotype is a picture of
chromosomes arranged in their pairs (46, 23 pairs in humans)
Male Female
Human HeredityAll egg cells carry an X chromosome,
half sperm cells carry an X, half a Y◦Leads to half combined becoming
either XY, or XXAutosomes are non-sex chromosomes,
labeled pairs 1-22 chromosomesSex chromosomes are the 23rd pair of
chromosomes
Human HeredityTo 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
offspringWhite forelock (lock of hair just above
the forehead) is dominant
Human Heredity
Human HeredityMany 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 pattern
Symptoms
Cystic fibrosis Simple (autosomal) recessive
Mucus in lungs, digestive tract, liver
Sickle cell disease
Simple (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 -35-50
Colorblindness
Sex-linked recessive Difficulty discerning colors
Hemophilia Sex-linked recessive Blood doesn’t clot correctly, missing Factor VIII
Down syndrome (Trisomy 21)
Trisomy (3 chromosomes instead of 2)
Mild to severe mental retardation
Human HeredityCystic fibrosis leads to serious
digestive problems◦Thick, heavy mucus clogs lungs and
breathing passages◦Since it is recessive, must have 2
alleles to be affectedSickle cell disease leads to
bent/twisted shape of RBCs (can clog in capillaries)◦Heterozygous individuals are
generally healthy and more resistant to malaria
Chromosomal DisordersDown 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 diseasesSex chromosome nondisjunction
◦Turner syndrome (X): sterile◦Klinefelter’s syndrome (XXY): generally
sterile