Patterns of Inheritance

Key Concept Body Cells vs Gametes Gametes have half the number of chromosomes that body cells have. -- body cells are also called somatic -- germ cells develop into gametes and are found in the ovaries & testes -- gametes are sex cells : egg & sperm -- gametes are haploid -- human gametes have 23 chromosomes

Chromo - somes -- human body cells have 46 chromosomes (23 pairs ) -- for each pair, one chromosome comes from each parent -- these pairs are called homologous and have the same structure -- chromosome pairs 1-22 are called autosomes -- the last pair are ( either X or Y ) determine the sex in mammals and are called allosomes Females: XX Males: XY

Haploid vs. Diploid -- diploid ( body ) cells have two copies of each chromosome (2 n ) -- half from each parent -- haploid ( sex ) cells have one copy of each chromosome ( n ) --22 autosomes & 1 sex chromosome

Mitosis -- produces diploid cells from body ( somatic ) cells

Meiosis -- produces haploid sex cells ( gametes ) from diploid cells

Meiosis -- gametes go through two rounds of division in meiosis -- meiosis I and meiosis II have four phases similar to mitosis -- pairs of homologous chromosomes separate in meiosis I -- homologous chromosomes are similar but not identical -- sister chromatids divide in meiosis II -- sister chromatids are copies of the same chromosomes

Meiosis I -- Meiosis I occurs after DNA has been replicated (4 n ) -- Prophase I : nucleus breaks down ; duplicated chromosomes condense ; spindle forms -- Metaphase I : chromosomes align ; each side has “ X ” from both parents -- Anaphase I : homologous chromatids pulled to opposite ends -- Telophase I : spindle fibers breaks down ; cell undergoes cytokinesis (2 n ) Crossing over (chiasma) occurs; genes are exchanged between homologous chromosomes Tetrads Cells are now haploid (1 of each “X”), but still duplicated (sister chromatids) and are 2n

Meiosis II -- Meiosis II : DNA is not replicated between I & II -- Prophase II : nucleus breaks down ; spindle forms ; centrioles migrate (2 n ) -- Metaphase II : chromatids align along the equator -- Anaphase II : sister chromatids pulled to opposite ends -- Telophase II : spindle fibers breaks down ; nuclear membranes reform ; cell undergoes cytokinesis ( n )

Meiosis vs Mitosis Meiosis differs from mitosis in significant ways : MitosisMeiosis Produces genetically identical cells. Produces genetically unique cells. Results in diploid cells.Results in haploid cells. Takes place throughout an organism’s lifetime. Takes place only at certain times in organism’s life cycle. Involved in asexual reproduction. Involved in sexual reproduction.

Gameto - genesis Gametogenesis is the production of gametes. -- differs between females & males -- males : sperm become streamline & motile ; only contribute DNA -- females : eggs contribute DNA, cytoplasm & organelles ; during meiosis one cell gets most of the contents ; the other cell forms polar bodies

Key Concept Mendel Mendel ’ s research showed that traits are inherited as discrete units. -- Mendel showed that traits are inherited as discrete units -- traits were previously thought as blended -- traits are distinguishing characteristics that are inherited -- father of genetics ( study of biological inheritance patterns ) “either –or”

Mendel ’ s pea plants Mendel made three key decisions in his experiments. -- used purebred plants -- control over breeding -- observation of seven “ either - or ” traits Mendel used pollen to fertilize selected pea plants. -- interrupted self pollination process by removing male flower parts

Mendel Mendel allowed the resulting plants to self - pollinate. -- among F 1 generation, all plants had purple flowers ( F 1 plants are all heterozygous ) -- among the F 2 generation, some plants had purple & some had white

Mendel Mendel observed patterns in the first and second generations of his crosses. Don’t copy this chart down!

Mendel Video : https :// www. youtube. com / watch ? v = G TiOETaZg 4 w Mendel drew three important conclusions : 1) Traits are inherited as discrete units. 2) Organisms inherit two copies of each gene, one from each parent. 3) The two copies segregate during gamete formation. * 2 & 3 are called the law of segregation

Mendel Law of Segregation : homologous chromosomes randomly separate into two different cells ; occurs during anaphase I. Law of Independent Assortment : traits are randomly distributed into gametes independently of each other during meiosis.

Key Concept Genes Genes encode proteins that produce a diverse range of traits. -- section of DNA that directs a cell to make a certain protein -- each gene has a specific position ( locus ) on a pair of homologous chromosomes -- the same gene can have many versions ( ie ; wrinkled or round, purple or white flower, etc )

Allele -- any alternative form of a gene at a specific locus on a chromosome -- each parent donates one allele for every gene -- homozygous describes two of the same alleles -- heterozygous describes two different alleles

Genotype vs Pheno - type -- genes influence the development of traits -- all of an organism ’ s genetic material is called the genome -- a genotype refers to the make up of a specific set of genes ( ie ; Rr ) -- a phenotype is the physical expression of a trait ( ie ; round )

Alleles Use Letters -- a dominant allele is expressed as the phenotype when at least one allele is dominant ( Rr or RR ) -- a recessive allele is expressed as a phenotype only when two recessive alleles are present ( rr ) -- dominant alleles are uppercase ; recessive are lowercase

Alleles Use Letters -- both homozygous dominant & heterozygous genotypes yield a dominant phenotype -- most traits occur in a range and do not follow simple dominant - recessive patterns

Key Concept Punnett Squares The inheritance of traits follows the rules of probability. -- grid system for predicting possible genotypes of offspring

Mono - hyrid Cross -- examines only one specific trait -- homozygous dominant ( FF ) -- homozygous recessive ( ff ) -- heterozygous ( Ff ) All heterozygous: Ff 1:2:1 Homozygous Dom. Heterozygous Dom. Homozygous Res.

Dihybrid Cross -- involves a cross of two traits -- heterozygous dihybrid cross yields a ratio of 9:3:3:1 ratio of phenotypes -- Mendel ’ s dihybrid crosses led to his 2 nd law of independent assortment -- allele pairs independently separate during meiosis

Heredity Patterns -- heredity patterns can be calculated -- probability is the likelihood that something will happen -- probability applies to random events such as meiosis and fertilization

Key Concept Sexual Reproductio n Independent assortment & crossing over during meiosis result in diversity. -- sexual reproduction creates unique combinations of genes -- caused by random fertilization of gametes -- unique phenotypes may give reproductive advantage to some organisms

Crossing Over Crossing over is the exchange of chromosomes segments between homologous chromosomes. -- occurs during prophase of meiosis I -- results in new gene combinations

Crossing Over Chromosomes contain many genes. -- the farther apart two genes are located, the more likely they are to be separated -- genes close together tend to be inherited together ( genetic linkage )

Key Concept Phenotype The chromosomes where genes are located affect expression of traits. -- two copies of autosomal genes affect phenotype -- Mendel ’ s rules of inheritance apply to autosomal genetic disorders -- a heterozygote for a recessive disorder is a carrier -- disorders caused by dominant alleles are uncommon

-- Phenotype -- example of a recessive disorder (dominant)

Sex - linked Traits Genes on sex chromosomes are called sex - linked genes. -- in mammals, Y chromosomes are responsible for male traits ; X chromosomes affect many traits

Males Male mammals have an XY genotype. -- all of a male ’ s sex linked genes are expressed -- males have no second copies of sex - linked genes

Females Female mammals have an XX genotype. -- expression of sex - linked genes is similar to autosomal genes in females -- X chromosomes inactivation “ turns off ” one X chromosome

Key Concept Phenotype Phenotype is affected by many different factors. -- depend on interactions of alleles -- incomplete dominance : neither allele is completely dom / rec -- heterozygous phenotype is intermediate between two homozygous phenotypes ( blended )

Co - dominant Ex ) blood type ABO -- codominant alleles are neither dominant or recessive. -- codominant alleles will both be completely expressed -- genes may have more than 2 alleles

Polygenic Traits Ex ) eye color -- polygenic traits are produced by two or more genes -- genes interact to produce one trait order of dominance : brown > green > blue

Epistatic genes Ex ) eye color -- epistatic genes can interfere with other genes -- the gene for albinism masks and is dominant over all other genes for skin or fur color

Environmen t Ex ) sea turtles -- the environment interacts with genotype -- phenotype is a combination of genotype and the environment -- the sex of sea turtles depends on both genes and the environment -- height is strongly affected by the environment

Key Concept Gene Linkage Genes can be mapped to specific locations on chromosomes. -- was explained by experiments on fruit flies -- Morgan found linked traits on the same chromosome -- chromosomes ( not genes ) are independently assorted during meiosis

Linkage Maps -- linkage maps estimate distances between genes -- the closer two genes are, the more likely they ’ re inherited together -- cross - over frequencies are related to distances between genes

Linkage Maps Cross - over frequencies can be converted into map units

Key Concept A combination of methods is used to study human genetics. -- basic principles are the same in all sexually reproducing organisms -- inheritance of many human traits is complex -- single gene traits are important in understanding human genetics

Sex - linked Disorders Females can carry sex - linked genetic disorders. -- males ( XY ) express all of their sex linked genes -- expression of disorder depends on which parent carries the allele and the sex of the child

Pedigree A pedigree is a chart for tracing genes in a family. -- phenotypes are used to infer genotypes on a pedigree -- autosomal genes show different patterns than do sex - linked genes

Pedigree If the phenotype is more common in males, the gene is likely sex - linked.

Karyotype -- is a picture of all the chromosomes in a cell -- can show changes in chromosomes -- deletion or loss of a chromosome -- duplication or extra chromosome

Example

Color blindness is a genetic disorder. It is due to a recessive X - linked allele ( X c ). It is possible for two parents without colorblindness to have a child with the condition. Why is the condition more common in males ? XCXC XcXc XCXC XC XCXC XC XC XcXC Xc YXCYXCYXcYXcY

Test Cross Some biology students wanted to determine whether a pair of hamsters purchased at the pet store were homozygous dominant ( HH ) or heterozygous ( Hh ) for curly hair. If the hamster is homozygous dominant, all offspring will have curly hair. If the hamster is heterozygous, 50% will have curly hair. HH hHh h Hh h hh hHhhh