Mendel and the Gene Idea Chapter 4.3 Mendel and the Gene Idea

Table 14.1 Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants

These alternative versions of a gene are now called alleles 4.3.1 Mendel’s Model First: alternative versions of genes account for variations in inherited characters For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers These alternative versions of a gene are now called alleles Each gene resides at a specific locus on a specific chromosome © 2011 Pearson Education, Inc.

Figure 14.4 Second: for each character, an organism inherits two alleles, one from each parent Allele for purple flowers Pair of homologous chromosomes Locus for flower-color gene Figure 14.4 Alleles, alternative versions of a gene. Allele for white flowers

Third: if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance © 2011 Pearson Education, Inc.

Fourth (now known as the law of segregation): the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes © 2011 Pearson Education, Inc.

Useful Genetic Vocabulary 4.3.1 An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup © 2011 Pearson Education, Inc.

PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Figure 14.6 Phenotype Genotype Purple PP (homozygous) 1 3 Pp (heterozygous) Purple 2 Pp (heterozygous) Purple Figure 14.6 Phenotype versus genotype. pp (homozygous) 1 White 1 Ratio 3:1 Ratio 1:2:1

Dominant phenotype, unknown genotype: PP or Pp? Figure 14.7 TECHNIQUE Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp Predictions If purple-flowered parent is PP or If purple-flowered parent is Pp Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs Figure 14.7 Research Method: The Testcross P p Pp Pp pp pp RESULTS or All offspring purple 1/2 offspring purple and 1/2 offspring white

Using a dihybrid cross, Mendel developed the law of independent assortment The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome Genes located near each other on the same chromosome tend to be inherited together © 2011 Pearson Education, Inc.

Hypothesis of dependent assortment Figure 14.8 EXPERIMENT YYRR P Generation yyrr Gametes YR yr F1 Generation YyRr Predictions Hypothesis of dependent assortment Hypothesis of independent assortment Sperm Predicted offspring of F2 generation or 1/4 YR 1/4 Yr 1/4 yR 1/4 yr Sperm 1/2 YR 1/2 yr 1/4 YR YYRR YYRr YyRR YyRr 1/2 YR YYRR YyRr 1/4 Yr Eggs YYRr YYrr YyRr Yyrr Eggs Figure 14.8 Inquiry: Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character? 1/2 yr YyRr yyrr 1/4 yR YyRR YyRr yyRR yyRr 3/4 1/4 1/4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9/16 3/16 3/16 1/16 Phenotypic ratio 9:3:3:1 RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1

Segregation of alleles into eggs Segregation of alleles into sperm Figure 14.9 Rr  Rr Segregation of alleles into eggs Segregation of alleles into sperm Sperm 1/2 R 1/2 r R R 1/2 R R r Figure 14.9 Segregation of alleles and fertilization as chance events. 1/4 1/4 Eggs r r R r 1/2 r 1/4 1/4

Multiple Alleles 4.3.3 Most genes exist in populations in more than two allelic forms For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i. The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither © 2011 Pearson Education, Inc.

BbEe BbEe Sperm 1/4 1/4 1/4 1/4 Eggs 1/4 1/4 1/4 1/4 9 : 3 : 4 BE bE Figure 14.12 BbEe BbEe Sperm 1/4 1/4 BE 1/4 1/4 bE Be be Eggs 1/4 BE BBEE BbEE BBEe BbEe 1/4 bE BbEE bbEE BbEe bbEe 1/4 Be BBEe BbEe BBee Bbee Figure 14.12 An example of epistasis. 1/4 be BbEe bbEe Bbee bbee 9 : 3 : 4

Polygenic Inheritance Quantitative characters are those that vary in the population along a continuum Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype Skin color in humans is an example of polygenic inheritance © 2011 Pearson Education, Inc.

Number of dark-skin alleles: 1 2 3 4 5 6 Figure 14.13 AaBbCc AaBbCc Sperm 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 Eggs 1/8 1/8 Figure 14.13 A simplified model for polygenic inheritance of skin color. 1/8 1/8 Phenotypes: 1/64 6/64 15/64 20/64 15/64 6/64 1/64 Number of dark-skin alleles: 1 2 3 4 5 6

Degrees of Dominance 4.3.3 Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways © 2011 Pearson Education, Inc.

The Testcross The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual If any offspring display the recessive phenotype, the mystery parent must be heterozygous © 2011 Pearson Education, Inc.

4.3.4 Blood Type Blood Type is a codominant trait (both A and B can be expressed) Type O: oo (no A or B) – universal Donor Type A: AA or AO Type B: BB or BO Type AB: AB, has both – Universal Acceptor + and - is a 3rd gene, negative people can only accept negative blood, positive can take either type (OO -) is universal donor

(a) The three alleles for the ABO blood groups and their carbohydrates Figure 14.11 (a) The three alleles for the ABO blood groups and their carbohydrates Allele IA IB i Carbohydrate A B none (b) Blood group genotypes and phenotypes Genotype IAIA or IAi IBIB or IBi IAIB ii Figure 14.11 Multiple alleles for the ABO blood groups. Red blood cell appearance Phenotype (blood group) A B AB O

Practice Questions Suppose a father of blood type A and a mother of blood type B have a child of type O. What blood types are possible in their subsequent children? Suppose a father and mother claim they have been given the wrong baby at the hospital. Both parents are blood type A. The baby they have been given is blood type O. What evidence bearing on this case does this fact have?

Is a widow’s peak a dominant or recessive trait? b) Figure 14.15 Key Male Female Affected male Affected female Mating Offspring 1st generation Ff Ff ff Ff 1st generation Ww ww ww Ww 2nd generation 2nd generation FF or Ff ff ff Ff Ff ff Ww ww ww Ww Ww ww 3rd generation 3rd generation ff FF or Ff WW or Ww ww Figure 14.15 Pedigree analysis. Widow’s peak No widow’s peak Attached earlobe Free earlobe (a) Is a widow’s peak a dominant or recessive trait? b) Is an attached earlobe a dominant or recessive trait?

Mutagens are agents that cause gene mutations such as: 3.4.U9 Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer. A mutation is a change in an organisms genetic code. A Gene mutation is a change in the nucleotide sequence of a section of DNA coding for a particular feature Mutagens are agents that cause gene mutations such as: chemicals that cause mutations (like some found in tobacco smoke) are referred to as carcinogens Alleles of a gene are similar, but have variations in the base sequence. New alleles are created by gene mutation. https://commons.wikimedia.org/wiki/File:Papierosa_1_ubt_0069.jpeg

nuclear bombing of Hiroshima 3.4.A4 Consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl. nuclear bombing of Hiroshima Elevated rate of Leukemia (with the greatest impact in children and young adults) Elevated rates of other cancers No evidence of stillbirth or mutations in the children of those exposed to radiation https://upload.wikimedia.org/wikipedia/commons/f/f6/Hiroshima_girl.jpg https://upload.wikimedia.org/wikipedia/commons/e/e9/The_patient%27s_skin_is_burned_in_a_pattern_corresponding_to_the_dark_portions_of_a_kimono_-_NARA_-_519686.jpg http://i.telegraph.co.uk/multimedia/archive/02446/hiroshima-bomb_2446747b.jpg

accident at Chernobyl nuclear power station 3.4.A4 Consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl. accident at Chernobyl nuclear power station Radioactive isotopes released into the environment exposing humans and other organisms to potentially dangerous levels of radiation. nuclear bombing of Hiroshima http://i.telegraph.co.uk/multimedia/archive/02446/hiroshima-bomb_2446747b.jpg https://upload.wikimedia.org/wikipedia/commons/1/16/VOA_Markosian_-_Chernobyl02.jpg